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/SCsub

@@ -14,12 +14,14 @@ common_linuxbsd = [
 
 if "x11" in env and env["x11"]:
     common_linuxbsd += [
-        "context_gl_x11.cpp",
+        "gl_manager_x11.cpp",
         "detect_prime_x11.cpp",
         "display_server_x11.cpp",
         "key_mapping_x11.cpp",
     ]
 
+#"context_gl_x11.cpp",
+
 if "vulkan" in env and env["vulkan"]:
     common_linuxbsd.append("vulkan_context_x11.cpp")