Resolves https://github.com/flutter/flutter/issues/130947.
- Allow for setting the text renderer (`TextRenderContext`) when the
`AiksContext` is built. Previously, the text renderer was selected at
build time through linking a `TextRenderContext::Create` symbol.
- Support using Impeller without a text backend. Throw a clear error
when trying to render text if no text backend is present.
- Don't track the Impeller context in `TextRenderContext`. Just let the
renderer supply its context when generating atlases.
- Allow for overriding the `TextRenderContext` for individual Aiks
playground tests/goldens.
cdab07b84a