Fixes #225.

Fixes #224

Fixes #222

Also does some cleanup, including switching to use
a builder and re-naming a few classes.

Otherwise it's likely our new Bitmaps will be 
immediately evicted.

Hack for #219

Fixes #171.

Fixes #217.

Fixes #216.

This does not make GifDecoder thread safe in that
it doesn't guarantee that a decoder will decode
a particular frame, but should keep state
consistent and prevent crashes from concurrent
writes to buffers.

Fixes #212.

Progress toward #212.

Fixes #214

Fixes #145.

Constantly posting resource recycles can cause
recycled data (Bitmaps) to not be available 
synchronously which can lead to unnecessary 
allocations.

Fixes #211

Progress toward #213.

More work toward #212.

Adds assertions for #212.

Fixes #210

Fixes #205.

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.

Fixes #207.

Fixes #206