WebPAnimEncoder: choose max diff for framerect based on quality.
WebPAnimEncoder: choose max diff for framerect based on quality.

We pick a mapping with quality 0 mapping to max diff 32, to quality 100
mapping to max_diff 1.

For 6k GIF image set, this improves compression by:
4% at quality 0
0.05% at quality 75

Benefits the MovingThumbnailer test videos too.

Change-Id: I6838ce864d41e1e65311d26b9b8115a12390a253

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp