st/mesa: make winsys fbo sRGB-capable when supported

st/mesa: make winsys fbo sRGB-capable when supported

The texture formats of winsys fbo are always linear becase the st manager
(st/dri for example) could not know the colorspace used.  But it does not mean
that we cannot make the fbo sRGB-capable.  By

 - setting rb->Visual.sRGBCapable to GL_TRUE when the pipe driver supports the
   format in sRGB colorspace,
 - giving rb an sRGB internal format, and
 - updating code to check rb->Format instead of strb->texture->format,

we should be good.

Fixed bug 75226 for at least llvmpipe and ilo, with no piglit regression.

v2: do not set rb->Visual.sRGBCapable for GLES contexts to avoid surprises

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75226

Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>