Fix HiDPI platform view transforms in CanvasKit

[cbenhagen]

Description

Completing the fix started in #22945

/cc @hterkelsen @yjbanov

Related Issues

Fixes flutter/flutter#61290

Tests

I added the following tests: TBD

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[yjbanov]

Can we initialize headTransform to Matrix4.diagonal3Values(inverseScale, inverseScale, 1) instead of applying the scaled transform at the end? This should eliminate 2 extra matrix allocations (Matrix4.identity() and scaleMatrix.multiplied).

[cbenhagen]

I am not sure if I fully understand what you mean. Please feel free to take over this PR and commit changes to my branch. I must confess, I merely applied the fix applied in #22945 without deep understanding of how this all fits together.

[yjbanov]

The headTransform was IDENTITY x T1 x T2 x ... x Tk x inverseScale. After this change it becomes inverseScale x IDENTITY x T1 x T2 x ... x Tk, which means we can remove IDENTITY and start with inverseScale. But the devil is probably in the detail. For example, if there's a clipping transform we reset it back to IDENTITY. I'm wondering if applying the scale at the end works in the presence of clips at all. If I'm reading the code correctly, it seems it doesn't?

Example: T1 > CLIP1 > T2 becomes T1 > CLIP1 > inverseScale > T2, but should probably be inverseScale > T1 > CLIP1 > T2. IIUC the inverseScale is there to undo the top-level transform that the flutter framework injects in https://github.com/flutter/flutter/blob/47bad77287effb3467f27937884036f51da6e325/packages/flutter/lib/src/rendering/view.dart#L144, so in order to undo that we should apply the inverse at the top level too.

I can take it over if you'd like, but also happy to provide guidance. I think we also need @hterkelsen's thoughts on this to make sure we're not breaking it altogether :)

[cbenhagen]

Yes please take it over. Feel free to close this and create a separate PR or just push to my branch.

[yjbanov]

SGTM. I'll start a new PR. Thanks for discovering the bug!

[yjbanov]

Closing the PR to start a new one.