Fix artefacts at right border of scaled bitmap subtitles by clearing
the destination to 0 first if the width is not a multiple of 8.
This is necessary because some of the optimized sub drawing functions
always draw pixels in multiples of 8, so we cannot rely on any "dirt"
in the "unused" borders being ignored.
Note that due to the way rendering works, both alpha and the image need
to be cleared.
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@33230 b3059339-0415-0410-9bf9-f77b7e298cf2