Refactor to improve performance around image masks (bug 1941562)

* add support for directly writing masks into `rgbaBuf` if their size matches
* add support for writing into `rgbaBuf` to `resizeImageMask` to avoid extra
  allocs/copies
* respect `actualHeight` to avoid unnecessary work on non-emitted rows
* mark more operations as `internal`

This changes the path for what I believe is the common case for masks:
a mask to add transparency to the accompanying opaque image, both being
equal in size.
The other paths are not meaninfully unchanged.
That increases my confidence as these new paths can be easily tested
with a PNG with transparency.

Author: wooorm