Draft: backends/standalone/x11: use sRGB framebuffers

This makes blending happen in linear space, which matches how blending works with color management in the Wayland session.

Note that I couldn't actually test the EGL backend code; when I tried to run it, kwin_x11 logged something about the shared context being wrong. The glx code works though, so I assume the egl one should too

Edit: this should only be merged once Wayland does color management by default too