Working on https://github.com/flutter/engine/pull/42415 , I found that it was difficult to customize the buffer layout as the interleaved layout is implicitly confiured in the backends. Rather than bake another built in layout, I've pulled (most) information about buffer layout into the generated headers so it is explicitly confiured, which should allow easier customization as the backend has fewer choices to make.
TBD whether or not we need to do something weird for GLES since stride has a different meaning there...
Work towards https://github.com/flutter/flutter/issues/116168
Makes the index buffer optional. For calls that were just filling the index buffer with e.g. `std::iota`, it was wasteful. @jonahwilliams has also found cases where the index buffer will run out of room for drawPoints with lots of points.
Fixes https://github.com/flutter/flutter/issues/126572
