rework a bit how we manage EGL extensions

- don't advertise extensions that are not supported
  by any implementation

- remove EGL_ANDROID_swap_rectangle which is not
  implemented by anybody and confuses people

- add some comments about mandatory extensions

Bug: 5428001
Change-Id: Id8dc48116ac1d1eb79ec9ef55d03e29d4257c1f3
17 files changed