vwebp: fix incorrect clipping w/NO_BLEND
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

Change-Id: I2fc4f5be99057e0bf87d8fedec57b06859b070bd

webmproject/libwebp,Aexyn/libwebp,Aexyn/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,Aexyn/libwebp,ttyangf/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,redengineer/libwebp,ttyangf/libwebp,Aexyn/libwebp,redengineer/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,redengineer/libwebp,Aexyn/libwebp,PKRoma/libwebp,redengineer/libwebp,redengineer/libwebp