Fix subtitles when no size received

[lukasf]

This fixes subtitles in the video reported by @ramtinak. If we do not get any size info for bitmap subs, we optimistically assume the size is same as the video size. Can't do any better than that.

[brabebhin]

Both VLC and PotPlayer seem to override the color.
Perhaps we should override the blacks with something else? I am not sure how to do it safely though, we'd have to keep black outlines but not black text. Difficult.