GRAPHICS: OpenGL 3.2 context creation (updated)

 - warning is used instead of error (oops).
 - _gl3context renamed to _gl3 to avoid name confusion with _glcontext.
 - better warning messages to notify of gl2.1 context creation.
 - TODO: not all SDL_GL_CreateContext areas are properly handled,
         and they really should be. Needs to be done.

Note: this commit will be later squashed down.
GRAPHICS: OpenGL 3.x context (updated)

FSAA context creation checks to see which OpenGL version to
create a context for.