add WebPMuxSetCanvasSize() to the mux API

previously, the final canvas size was adjusted tightly from the
animation frames. Now, it can be specified separately (to be larger, in particular).

calling WebPMuxSetCanvasSize(mux, 0, 0) triggers the 'adjust tightly' behaviour.
This can be useful after calling WebPMuxCreate() if further image addition
is expected.

-> Fixed gif2webp accordingly.

also: made WebPMuxAssemble() more robust by systematically zero-ing WebPData.

Change-Id: Ib4f7eac372cf9dbf6e25cd686a77960e386a0b7f