Improve snapshot tests

[PoignardAzur]

Right now the assert_render_snapshot! macro and the underlying code are held together with duck-tape.

Some possible improvements:

• Better error messages when a snapshot test fails.
• Some sort of cargo-insta style UI tool that shows you the image and asks you to validate one or the other.

[PoignardAzur]

(Copied from some notes I took during the Winit port:)

The basic idea with snapshot tests is simple: we create a widget tree that we want to test; we lay them out and render them to a bitmap; then we save that bitmap to the repository. In future tests, we do the process again and check that the bitmap hasn't changed. If it has, it likely indicates either a bug or that our rendering code changed.

That's great in theory, but in practice these tests tend to be flaky. There are several things that can introduce false negatives in CI:

• Non-deterministic floating point computations when rendering the bitmap.
• Platform-dependent themes.
• Platform-dependent system fonts.

To fix snapshot tests, we need be able to reliably avoid these issues.

• Deterministic rendering needs to be addressed directly in Vello.
• Themes need to be addressed by scouring the codebase for platform-dependent paint code and removing it. Any platform differences in rendering should be reflected in attributes given to the widgets, not in the widget code.
• Fonts should probably be addressed in Parley code.