AnimEncoder: Support progress hook and user data.
AnimEncoder: Support progress hook and user data.

Pass them along to internal 'pic' object, so that progress can be reported back
and user data can also be inspected.

Change-Id: Idb5d0d4a76d07283d704a86c5892e1ad7bda09fa

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