Added new extensions

The new extensions are:
- GL_OES_framebuffer_object (with extra exports)
- GL_OES_depth24
- GL_OES_depth32
- GL_EXT_color_buffer_half_float

These were mostly supported already with the GLES3 related changes,
so only a few minor changes were required.

Change-Id: Iab6e94043cfdabb8ad74e526f44f7bd9d57c267f
Reviewed-on: https://swiftshader-review.googlesource.com/4342
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>