opengl/glshader: use gamma 2.2 for sRGB render targets

The sRGB global colordescription uses gamma 2.2, so this function also has to do
it, or the shader will end up doing a colorspace conversion where there shouldn't
be one.
This also meams that HDR content will be wrongly encoded, but fixing that while
also dealing with sRGB content correctly requires a lot more invasive changes, in
KWin and in the screenshot and screencast APIs.

BUG: 478967


(cherry picked from commit c1e09f65)