vwebp: clear canvas during resize w/o animation
vwebp: clear canvas during resize w/o animation

this corrects the checkboard pattern displayed with transparent images

Change-Id: I5f46dbc9fa3893d61f5f1d4fda643ac030238f94

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