mesa: Add extension enables for framebuffer fetch extensions.

mesa: Add extension enables for framebuffer fetch extensions.

This allows drivers to expose EXT_shader_framebuffer_fetch in GLES2+
contexts if desired.  Note that this adds boolean flags for two MESA
extensions, but only the EXT GLES-only extension is exposed for the
moment, see the cover letter of this series [1] for the rationale.

[1] https://lists.freedesktop.org/archives/mesa-dev/2016-July/124028.html

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>