Rather than changing the size of the child elements (relaying out text
and potentially changing glyph shape and position due to pixel
snapping), apply the stretch effect as a compositing effect - render the
children to a texture and stretch the texture. If we end up using an
image shader to apply an custom shader we'll need to do this anyway.

Fixes https://github.com/flutter/flutter/issues/129528

### Before


https://github.com/flutter/flutter/assets/8975114/16e9eb57-f864-4093-b4a4-461082b89b43

### After


https://github.com/flutter/flutter/assets/8975114/14cf0167-8922-4876-a325-e3bc154b084f