Provide a relative, per-image limit to the amount of memory
that's used to cache decoded image frames. Adds an overridable default
that developers can set to control how much memory images are allowed
to use decoded vs undecoded. The cap is set in flutter/flutter#22452.
Note that required frames are always cached regardless of the ratio cap,
because they're currently necessary for the GIF to animate. Previously
cached unessential frames are not cleared in response to the cache
hitting or exceeding the cap.
Addresses #20998 and #14344.
This is the first step to support animated GIFs: flutter/flutter#204
TBD in following CLs:
* Implement Codec.getNextFrame.
* Add Framework side support to run animations.
