[Impeller] Reland: hash less stuff per frame.

[jonahwilliams]

Reverted due to missing glyph when rolling into the framework. Problem was that we should be always using the unrounded rounded scale to compute subpixel position but we sometimes used the rounded scale.

[flutter-dashboard[bot]]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[gaaclarke]

Note: 31b7bdc is where this was merged last.

[jonahwilliams]

Positive results on text performance

https://flutter-flutter-perf.skia.org/e/?keys=Xba015abd0041910c85da436f62fbd73d&selected=commit%3D43118%26name%3D%252Carch%253Dintel%252Cbranch%253Dmaster%252Cconfig%253Ddefault%252Cdevice_type%253Dmokey%252Cdevice_version%253Dnone%252Chost_type%253Dlinux%252Csub_result%253D90th_percentile_frame_rasterizer_time_millis%252Ctest%253Dfullscreen_textfield_perf__e2e_summary%252C&xbaroffset=43118