SetRenderTargetType is used to configure the backingstore producer on the compositor, but the backingstore types available to any given compositor are limited to the specific graphics backend in use: Software, GL, Metal, or Vulkan.
This moves SetRenderTargetType to EmbedderTestCompositor and its subclasses and adds RenderTargetType validation. A follow-up patch will refactor EmbedderTestBackingStoreProducer into backend-specific subclasses.
For OpenGL backingstore producers, the egl_context_ from EmbedderTestContext (which is actually set in the EmbedderTestContextGL subclass and should live there, but that's a separate cleanup) is required in SetRenderTargetType, so we now inject it into EmbedderTestCompositor in the constructor so it's available when needed.
Issue: https://github.com/flutter/flutter/issues/158998
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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