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>

platform/linuxbsd/detect.py

@@ -381,8 +381,8 @@ def configure(env):
             # No pkgconfig file for glslang so far
             env.Append(LIBS=["glslang", "SPIRV"])
 
-        # env.Append(CPPDEFINES=['OPENGL_ENABLED'])
-        env.Append(LIBS=["GL"])
+    env.Append(CPPDEFINES=["OPENGL_ENABLED"])
+    env.Append(LIBS=["GL"])
 
     env.Append(LIBS=["pthread"])