[Impeller] port clip stack fixes to new canvas.

[jonahwilliams]

From c99eda4

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[jonahwilliams]

When doing the clip replay for a bdf, what value should be used as the clip depth comparison? @bdero

[bdero]

Apologies for the late reply. When doing a clip restore:

• The MSAA backdrop should be the very first thing drawn, but I set the clip depth to std::numeric_limits<uint32_t>::max() anyway. This helps to ensure we're not unnecessarily writing depth for the MSAA backdrop (the depth/stencil are disabled with TextureContents::SetStencilEnabled(false)).
• Afterwards, each clip being replayed should have the same depth they had the first time they were being drawn.

[jonahwilliams]

Updated this to use max int32 for bdf restores, other flip backdrops (emulated advanced blend) use the entity depth