linux-capture: Load XSHM capture on EGL/X11

Unlike Xcomposite, the XSHM plugin does not use GLX code, and thus
can be used on when EGL renderer is used. It still is X11-specific
though, and shouldn't be used on Wayland.

Rework the obs_module_load() function of linux-capture to use a
switch statement, and load XSHM both on EGL/X11 and GLX/X11.

Fixes https://github.com/obsproject/obs-studio/issues/5122