+ property that one or more particular pixel values is transparent.
+
+ Vendor support:
+
+ SGI: - Supported on all systems since IRIX 4.0.
+ - Their version of the Motif toolkit renders menus into the overlay
+ visual, so that they don't cause exposure events on occluded
+ windows.
+ - 2 bit overlay plane with Indigo Entry graphics (e.g., Indy).
+ - 8 bit overlay on O2 with Indigo 24 graphics or better (e.g., O2).
+
+ HP: - Supported on workstations with CRX24 and CRX48Z graphics hardware.
+ - 8 bit or 24 bit overlay plane.
+ - The default visual is the overlay visual, with a transparent pixel.
+ That way, all Xlib programs draw into the overlay plane, and no
+ Xlib program causes exposures on occluded OpenGL windows.
+
+ IBM: - Supported on some graphics hardware (which?)
+
+ DEC: - Supported on some graphics hardware (which?)
+
+ SUN: - Some systems apparently implement it VERRRRRRY SLOWLY, so drawing
+ into the overlay plane is a performance killer (about as bad as
+ using the SHAPE extension.)
+