* Fix applyBoxFit's handling of fitWidth and fitHeight.

Previously, in `fitWidth` mode, if the input size had a wider aspect
ratio than the output size, `applyBoxFit` would make the source rect
taller than the input size in order to match the aspect ratio of the
destination rect.  Similarly, in `fitHeight` mode, if the input size
had a taller aspect ratio than the output size, `applyBoxFit` would
make the source rect wider than the input size in to match the aspect
ratio of the destination rect.  This is in contrast to all the other
modes, which never output a source rect that's larger than the input
size.

Most of the time this worked as intended (since attempting to blit
pixels that are outside the source image has no effect), however it
meant that if a user attempted to create a `BoxDecoration` that used
both `fitWidth` and `repeatY` (e.g. in an attempt to tile a background
image), the image would not actually appear to repeat, since the logic
in `paintImage` for determining the proper tiling stride is based on
the destination image size, meaning that the entire destination rect
would be covered in a single tile.

This change modifies `applyBoxFit` so that in `fitWidth` mode, if the
input size has a wider aspect ratio than the output size, it uses
formulas that are equivalent to `contain`, whereas if the input size
has a taller aspect ratio than the output size, it uses formulas that
are equivalent to `cover`.  And vice versa for `fitHeight` mode.  This
produces source and destination rects that match the behaviour
specified in https://api.flutter.dev/flutter/painting/BoxFit.html.

* Apply suggestions from code review
Co-authored-by: Michael Goderbauer <goderbauer@google.com>