Includes squashed changes of:
- m-c 1317907: Refactor FrameAnimator::GetCompositedFrame to be a bit simpler
- m-c 1351434: bugfix
- m-c 686905: Enable the pref image.mem.animated.discardable to allow discarding of animated images
https://bugzilla.mozilla.org/show_bug.cgi?id=1547792
Aspect Ratio handling simplified by using floating point integers:
- Multiplication of value (or inverse value) to a known side for Scaling
- No unequal equal values such as "4/3" vs "8/6" vs "20/15"
- Truly "Empty" aspect ratios, even if one dimension is not 0