Add convex path and non-AA paint to shader warm-up (#28614)

One of our important client's SKP shows that this could improve one of
their janky frame by 20ms. This also improves our
flutter_gallery__transition_perf's worst frame time by ~20ms.

On the other hand, 15ms has been added to the start-up latency. I guess
it's a little faster to compile the shader on the start-up because we're
compiling a lot of shaders there and the CPU cache must be hot.

## Related Issues

https://github.com/flutter/flutter/issues/813