Fix Xinerama's consolidated visual handling.
Formerly the code claimed it could only handle up to 256 visuals, which was true. Also true, but not explicitly stated, was that it could only handle visuals with VID < 256. If you have enough screens, and subsystems that add lots of visuals, you can easily run off the end. (Made worse because we allocate visual IDs from the same pool as XIDs.) If your app then chooses a visual > 256, then the Xinerama code would throw BadMatch on CreateColormap and your app wouldn't start. With this change, PanoramiXVisualTable is gone. Other subsystems that were using it as a translation table between each screen's visuals now use a PanoramiXTranslateVisual() helper.
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