Jonah Williams f73ae51144 [impeller] convert src over to src for solid color (flutter/engine#41351)
Related to https://github.com/flutter/flutter/issues/114402

At least in the flutter gallery test app, with https://github.com/flutter/gallery/pull/909 reverted - this significantly improves the performance in the Xcode frame debugger. If we wanted to do this for more color sources or filter effects, we need to do much more work to track texture opacity, but this seems like a reasonable and low cost first step.

### Before

![image](https://user-images.githubusercontent.com/8975114/233222371-f33d29ad-c556-4db0-b949-84777bb09307.png)

### After

![image](https://user-images.githubusercontent.com/8975114/233222350-0517d109-cabd-45af-a0af-06b95da2d026.png)
2023-04-20 01:07:33 +00:00
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