Ignore wholly translucent rows at the top and bottom of the subtitle page

Only create an overlay large enough for the remaining rows and only only ask
zvbi to render those rows.

One happy side effect is that now --vbi-text no longer outputs header text
for some teletext streams (although --vbi-text is still unusable imho).

Reducing the size of the overlay allows stutter free playback of teletext
subtitles on significantly slower hardware.