Automatic/scalable shader warm-up

Problem

Current status: https://github.com/flutter/flutter/issues/32170#issuecomment-1016927129 ---- Although Flutter's current [shader warm-up][1] process is theoretically able to eliminate all shader compilation jank, it requires too much expertise and time to find all draw operations for such process. We shall find a more automatic and scalable way of finding those draw operations to compose a custom warm-up process. That should allow average developers without much Skia knowledge to eliminate the shader compilation on their own. That should also save Flutter engineers a lot of time as they'll no longer have to manually analyze the janky frame. Note that this issue affects iOS devices more significantly than Android devices because Android has binary shader persistent cache while iOS doesn't. We have a staged plan to roll this out, and we're actively working on it. - [x] SkSL-based shader warmup - [ ] Test-based shader warmup - [ ] In the long term, we'd like to create a solution that requires 0 extra effort from developers, and handles all cases that may have never been encountered during any tests or warmups. For example, use the CPU backend when GPU backend needs warm-up. Related issue: flutter/flutter#813, flutter/flutter#31881 [1]: https://api.flutter.dev/flutter/painting/ShaderWarmUp-class.html