gles3: Prohibit set/get of GL_FRAMEBUFFER_SRGB.

gles3: Prohibit set/get of GL_FRAMEBUFFER_SRGB.

GLES 3 supports sRGB functionality, but it does not expose the
GL_FRAMEBUFFER_SRGB enable/disable bit.  Instead the implementation
is expected to behave as though that bit is always enabled.

This patch ensures that ctx->Color.sRGBEnabled (the internal variable
tracking GL_FRAMEBUFFER_SRGB) is initially true in GLES 2/3 contexts,
and that it cannot be modified through the GLES 3 API.

This is safe for GLES 2, since ctx->Color.sRGBEnabled has no effect on
non-sRGB formats, and GLES 2 doesn't support any sRGB formats.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>