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8167e14475
Implements partial repaint for Impeller. Fixes https://github.com/flutter/flutter/issues/124526 The new code that manages the damage regions is more or less a copy paste from the existing Skia implementation. Compared to Skia, there are a few differences: Normally Impeller wants to use the drawable as the resolve texture for the root MSAA pass. Unfortunately this will unconditonally clear that texture. Thus to do a partial repaint, we have to allocate a separate texture to resolve to and then blit into the drawable. The blit seems to take about 500ns for a full screen on an iPhone 13. That implies that partial repaint is likely not worth doing if the screen is significantly changed. Thus I've added code in compositor_context.cc that computes the percentage of width or height that is part of the dirty rect. Above a threshold of (abitrarily chosen) 70%, we just render as normal. This should mean there is only a very minor hit from performing the diff on screens that are highly changed. The other special case, is that sometimes we get damage rects that are empty - that is the drawable is already completely up to date with what we want to render. IN that case I shortcircuit all of the impeller code and just present immediately. I previously tried returning without a present but this resulted in Xcode reporting dropped frames. One caveat here is that if you use the XCode frame debugger and attempt to capture a frame where we early present, then it will claim it couldn't capture any command buffers (because we didn't create any). To facilitate all of this, I added some additonal plumbing so that the impeller surface can get the clip rect from the submit info. Additionally, rather than using a clip rect impeller will translate and then shrink the root surface texture. This reduces memory usage compared to just clippling.