RT-22845 uncovered 2 problems with the global blend mode used in Canvas rendering:

- There was a missing release of a resource used while implementing it causing huge VRAM image buffers to be leaked and reclaimed through slow garbage collection
- The operation is done on the entire canvas even when the transparent parts outside the bounds of the primitive have no effect.

The fix for RT-22845 fixed the first of those problems, but this bug is filed to track the second of those problems.