GLX: implement __GLX_VENDOR_LIBRARY_FILENAMES similar to __EGL_VENDOR_LIBRARY_FILENAMES #228
Description
Adds __GLX_VENDOR_LIBRARY_FILENAMES environment variable, similar to __EGL_VENDOR_LIBRARY_FILENAMES. Allows loading GLX vendors from JSON files. The JSON format is the same as for __EGL_VENDOR_LIBRARY_FILENAMES. Has lower priority than __GLX_VENDOR_LIBRARY_NAME.