First implementation with Linux display manager.
- Add single-threaded mode for EditorResourcePreview (needed for OpenGL).
Co-authored-by: clayjohn <claynjohn@gmail.com>
Co-authored-by: Fabio Alessandrelli <fabio.alessandrelli@gmail.com>
This code currently isn't compiled (and cannot compile).
We plan to re-add OpenGL ES-based renderer(s) in Godot 4.0 alongside Vulkan
(probably ES 3.0, possibly also a low-end ES 2.0), but the code will be quite
different so it's not relevant to keep this old Godot 3.2 code.
The `drivers/gles2` code from the `3.2` branch can be used as a reference for
a potential new implementation.
