vwebp: Fix bug when Dispose then NoBlend frames
vwebp: Fix bug when Dispose then NoBlend frames

The graphical bug happens when there is a frame disposed to background
color, followed by another frame that does not blend, and their areas
don't fully overlap. Only the previous frame clears its part of the
viewport. The fix consists in clearing the screen for the previous and
the current frame if needed.

Change-Id: I3425cf7297f0c7b2cf13a3a61b517cc0b1c031d8

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