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.