Eventually we might want to have an XCB-based EGL winsys. We already
have xlib-specific API in CoglRenderer (eg, to set a foreign display)
so the application needs to be able to specifically select between XCB
and XLIB.
This also removes the POWERVR part while renaming
COGL_HAS_EGL_PLATFORM_POWERVR_X11_SUPPORT to
COGL_HAS_EGL_PLATFORM_XLIB_SUPPORT because the winsys is equally
applicable to Mesa.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
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