Always decode gif frames in sequence.

Fixes #207.
Clean up GifDrawable resources more reliably.

Each time we call get() on a drawable resource, we
get a new Drawable. We call get() repeatedly on
resources when they are retrieved from either the
set of active resources or the in memory cache.
Each time we create a new GifDrawable it holds on
to one or two temporary Bitmaps outside it's
shared state to render the current frame and obey
the dispose_previous method. This change means we
more aggressively cleanup those resources when
we think each Drawable is no longer being used.
The side affect is that we may reset back to
the beginning of the Drawable in some
circumstances.

Cleanup in in memory resources makes it less
likely that frames would be retrieved from
in memory, so this also works toward #207.