mesa: add GL_OES_gpu_shader5 and GL_EXT_gpu_shader5 support

mesa: add GL_OES_gpu_shader5 and GL_EXT_gpu_shader5 support

The two extensions are identical, and are largely taking bits of already
existing desktop functionality. We continue to do a poor job of
supporting the 'precise' keyword, just like we do on desktop.

This passes the relevant dEQP tests that I could find.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>