Roughness is now set to 1 by default and albedo is now white,
even on meshes that do not have any materials defined.
This means there is no longer a visual difference between a
mesh with no materials defined and a mesh with a default
SpatialMaterial defined.
This commit adds a new rendering backend, GLES2, and adds a
project setting to enable it.
Currently this backend can only be used on the X11 platform,
but integrating into other platforms is planned.