[Impeller] Add a way to query the gfx backend type.

[bdero]

I've been trying to avoid this, but since the context type may vary through runtime decisions, we just need something like this to select which shader data to pull from the Dart GPU bundle at runtime. One alternative would be to add a special omni shader concept to the HAL, but that seems worse than just letting renderers key with a backend enum sometimes.

Cautioned against using it for cap checks in the doc string.

[flutter-dashboard]

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[chinmaygarde]

The test failures are real. The mock needs to be updated to account for the new API.

[auto-submit]

auto label is removed for flutter/engine, pr: 43837, due to - The status or check suite Linux Framework Smoke Tests has failed. Please fix the issues identified (or deflake) before re-applying this label.