xwayland/glx: Enable sRGB fbconfigs

We turn this on if the GL underneath us can enable GL_FRAMEBUFFER_SRGB.
We do try to generate both capable and incapable configs, which is to
keep llvmpipe working until the client side gets smarter about its srgb
5 jobs for glx-egl-srgb-fbconfigs in 6 minutes and 21 seconds (queued for 3 seconds)