vwebp: fix incorrect clipping w/NO_BLEND

when the previous frame does not specify dispose to background only the
current frame's rectangle should be cleared

related to bug #245

(cherry picked from commit 469ba2cdfdcaaabbc0993994243646c257b3526c)

Change-Id: I2fc4f5be99057e0bf87d8fedec57b06859b070bd
1 file changed