Sets the global alpha value associated with a OverlayRectangle.
Per- pixel alpha values are multiplied with this value. Valid values: 0 <= global_alpha <= 1; 1 to deactivate.
this must be writable, meaning its refcount must be 1. You can make the rectangles inside a OverlayComposition writable using make_writable or copy.
| this | |
| global_alpha |
Global alpha value (0 to 1.0) |