XCB/GLX: higher priority than Frame Buffer

Neither of them support YUV. But GLX is much more likely to work than
FB, and also fails in a safer way.
(cherry picked from commit b660648dff46b0ea05a73b094b8c306be128a741)

modules/video_output/xcb/glx.c

@@ -51,7 +51,7 @@ vlc_module_begin ()
     set_description (N_("GLX video output (XCB)"))
     set_category (CAT_VIDEO)
     set_subcategory (SUBCAT_VIDEO_VOUT)
-    set_capability ("vout display", 20)
+    set_capability ("vout display", 50)
     set_callbacks (Open, Close)
 
     add_shortcut ("xcb-glx")