Use OpenGL 3.3 core profile instead of compatibility profile

- Rename OpenGL to GLES3 in the source code per community feedback.
  - The renderer is still exposed as "OpenGL 3" to the user.
- Hide renderer selection dropdown until OpenGL support is more mature.
  - The renderer can still be changed in the Project Settings or using
    the `--rendering-driver opengl` command line argument.
- Remove commented out exporter code.
- Remove some OpenGL/DisplayServer-related debugging prints.

drivers/gles3/SCsub

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+
+Import("env")
+
+env.add_source_files(env.drivers_sources, "*.cpp")
+
+SConscript("shaders/SCsub")