This does several things:
- It adds CPU time on the IO thread, but avoids GPU time on the GPU
  thread.
- For images that are never drawn with mipmaps, it adds about 33%
  memory overhead. For images that are drawn with mipmaps, it saves
  an entire copy of the base level.
- It fixes https://github.com/flutter/flutter/issues/24517, which is
  a driver bug related to mip-mapping and cross-context images.

Overall, I think the tradeoff is good, but I'm curious to see what
benchmarks look like.