Fixes the following GCC 7 warnings:
```
core/cowdata.h:269:47: warning: 'alloc_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/error_macros.h:163:26: warning: 'nearest_point' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1579:5: warning: 'colormap_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1582:12: warning: 'size_height' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1590:23: warning: 'size_width' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/image.cpp:1599:29: warning: 'pixel_size' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/math/face3.cpp:207:15: warning: 'tri_max' may be used uninitialized in this function [-Wmaybe-uninitialized]
core/math/face3.cpp:209:15: warning: 'tri_min' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:665:22: warning: 'best_used_frame' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_storage_gles3.cpp:865:27: warning: 'blit_target' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/rasterizer_storage_gles3.cpp:980:29: warning: 'blit_target' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::frag_id' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::id' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gles3/shader_gles3.h:122:9: warning: '<anonymous>.ShaderGLES3::Version::vert_id' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/plugins/script_editor_plugin.cpp:1980:31: warning: 'se' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/scene_tree_dock.cpp:840:30: warning: 'new_node' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'a1' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'lll' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4259:9: warning: 'lul' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4260:9: warning: 'a2' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4261:9: warning: 'a3' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4265:3: warning: 'enable_lin' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4294:3: warning: 'enable_ang' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4311:34: warning: 'll' may be used uninitialized in this function [-Wmaybe-uninitialized]
editor/spatial_editor_gizmos.cpp:4311:34: warning: 'ul' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1655:47: warning: 'cone_dirs' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/3d/voxel_light_baker.cpp:1656:73: warning: 'cone_weights' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/gui/texture_progress.cpp:181:6: warning: 'cp' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/gui/texture_progress.cpp:181:6: warning: 'cq' may be used uninitialized in this function [-Wmaybe-uninitialized]
servers/physics/shape_sw.cpp:1056:19: warning: 'support_max' may be used uninitialized in this function [-Wmaybe-uninitialized]
```
Fixes the following Clang 7 warnings:
```
core/io/marshalls.cpp:872:10: warning: unused variable 'f' [-Wunused-variable]
core/ustring.cpp:1831:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
core/ustring.cpp:1832:2: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
drivers/gles3/rasterizer_gles3.cpp:82:24: warning: unused function '_gl_debug_print' [-Wunused-function,34]
main/main.cpp:118:13: warning: unused variable 'auto_build_solutions' [-Wunused-variable]
modules/csg/csg_gizmos.cpp:225:46: warning: 'current' may be used uninitialized in this function [-Wmaybe-uninitialized]
```
Fixes the following Clang 7 warnings:
```
drivers/gles3/rasterizer_scene_gles3.cpp:1260:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1280:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1281:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1282:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1286:5: warning: variable 'target' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
editor/editor_properties_array_dict.cpp:982:15: warning: variable 'change_index' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
editor/editor_properties.cpp:2655:6: warning: variable 'lt' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
editor/settings_config_dialog.cpp:367:11: warning: variable 'current_search_box' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
```
