[Impeller] Allow image rasterization/decoding before/without surface acquisition

[dnfield]

This is the same as #41059 with the Android stuff reverted.

This change provides an impeller::Context to the rasterizer so that it can help with snapshotting even if the surface has not been setup yet. It also makes the image decoding logic use that context directly instead of trying to acquire it through a surface, which the rasterizer may or may not have when the image decode is requested.

I originally (incorrectly) believed I'd have to implement the Android side for any of it to work. I think we should still land the Android changes, but this will make for a smaller cherry pick and just leave Android broken (the other patch didn't fix Android with GLES anyway).

Fixes flutter/flutter#124181

[auto-submit]

auto label is removed for flutter/engine, pr: 41168, due to - The status or check suite Mac Host Engine has failed. Please fix the issues identified (or deflake) before re-applying this label.

[dnfield]

Failures appear unrelated to this patch, updating branch...

[auto-submit]

auto label is removed for flutter/engine, pr: 41168, due to - The status or check suite Mac mac_host_engine has failed. Please fix the issues identified (or deflake) before re-applying this label.

[dnfield]

Failures on the mac_host_engine are unrelated to this pr, see #41179

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Changes reported for pull request #41168 at sha c804423