egl/x11: Stop allowing 24-bit visuals for 32-bit RGBA8888 configs

This effectively reverts commit 049f343e

Right now it is not possible to get the magic transparency bit on EGL/X11, stopping toolkits like GTK4 and applications like Firefox from purely using EGL on X11.

Further more the argument for the current behaviour is arguably rather weak: if there are still drivers that only support 32-bit RGBA configs, apps can use additional help to make sure they don't suffer a performance hit, such as _NET_WM_OPAQUE_REGION.

Stopping this behaviour makes EGL on X11 match closer what we do on Wayland, which has proven to work quite well. This should help more applications to finally transition from GLX to EGL, which is especially helpful to reduce maintaince overhead if both Wayland and X11 need to be supported in the future.

Closes #149

Alternative to !9989