CGame can draw twice in same draw frame, default Off
(this will help stereoscopic widget to display stable 3D image for moving unit)
Add Call-Out for LUAUI to trigger double drawing.
(this will be used by stereoscopic widget)
COMMANDS:
/doubledraw 1|0: enable/disable double drawing
Spring.IsDoubleDrawing(): check if double drawing is active
Spring.GetDoubleDrawIndex(): to make sure widget know which scene is double-drawed and which scene is from different frame