MainFrm: Check if pDVDI is initialized in SetupDVDChapters()
MainFrm: Fix chapter marks breaking if EC_DVD_DOMAIN_CHANGE is triggered on a file, this is consistent with EC_DVD_TITLE_CHANGE.
Prevent black lines from appearing on the sides of the video when resizing.

This happened mostly when resizing manually or when using auto-fit zoom.

Fixes #2423.