Add GLES2 2D renderer + Linux display manager

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>

drivers/SCsub

@@ -25,6 +25,11 @@ SConscript("winmidi/SCsub")
 # Graphics drivers
 if env["vulkan"]:
     SConscript("vulkan/SCsub")
+    SConscript("gles2/SCsub")
+    SConscript("gles_common/SCsub")
+    SConscript("gl_context/SCsub")
+else:
+    SConscript("dummy/SCsub")
 
 # Core dependencies
 SConscript("png/SCsub")