mesa/swrast: handle sRGB FBOs correctly (v2)

mesa/swrast: handle sRGB FBOs correctly (v2)

From reading EXT_texture_sRGB and EXT_framebuffer_sRGB and interactions
with FBO I've found that swrast is converting the sRGB values to linear for
blending when an sRGB texture is bound as an FBO. According to the spec
and further explained in the framebuffer_sRGB spec this behaviour is not
required unless the GL_FRAMEBUFFER_SRGB is enabled and the Visual/config
exposes GL_FRAMEBUFFER_SRGB_CAPABLE_EXT.

This patch fixes swrast to use a separate Fetch call for FBOs bound to
SRGB and avoid the conversions.

v2: export _mesa_get_texture_dimensions as per Brian's comments.

Signed-off-by: Dave Airlie <airlied@redhat.com>