[CP][Impeller] Update partial repaint to use a fullsize onscreen. (#161626) #162194
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The existing technique of offsetting a smaller texture is very vunerable to bugs in the renderer. Rather than this approach, we can allocate a new offscreen that is full sized and then blit a smaller region. To reduce the allocation costs, we can also set up a transients cache which will reuse this texture. In total, this should be more performant than the existing partial repaint (due to lack of continual re-allocation) at the cost of higher peak memory usage.
Fixes #140877 Fixes #160588 Fixes #156113