overlays: don't reduce ASS texture width unnecessarily

Fixes OpenELEC/OpenELEC.tv#3059 on GitHub, where an ASS subtitle
texture could become too narrow to fit subtitle bitmaps in full,
causing them to be inappropriately clipped. This issue has become
obvious lately due to changes in libass that make it output wider
(but fewer) images than it used to.
overlays: include padding in initial ASS texture width computation
Merge pull request #4518 from astiob/convert_quad

Clean up convert_quad slightly and fix subtitle corruption with new libass