enable: Create _mesa_set_framebuffer_srgb() function for use by meta ops.

enable: Create _mesa_set_framebuffer_srgb() function for use by meta ops.

GLES3 supports sRGB formats, but it does not support the
GL_FRAMEBUFFER_SRGB enable/disable flag (instead it behaves as if this
flag is always enabled).  Therefore, meta ops that need to disable
GL_FRAMEBUFFER_SRGB will need a backdoor mechanism to do so when the
API is GLES3.

We were already doing a similar thing for GL_MULTISAMPLE, which has
the same constraints.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>