Commit 87c13de6 added a fallback to video resolution if the subtitle
resolution is unknown. Apparently this fixed some broken files with
vobsubs.
This broke some DVB subtitles. Apparently .ts captures with 1920x1080
video resolution and 720x576 subtitles do exist. The sample at hand had
some streams with 720x576 resolution and no sub resolution set, and some
streams with 1920x1080 resolution and sub resolution set (both against
the same 1920x1080 video). My conclusion is that 720x576 is the only
reasonable fallback for DVB (but I can't be sure).
The fallback is removed for PGS too. I don't know about the PGS case; it
seems the sub resolution must always be set, so it shouldn't matter.
Fixes#1425.
cc32aa436e