Track selection: Don't override splitter choice if all tracks have the same priority.

Patch based on Kasper93's work.
Properties dialog: List tracks instead of pins when possible.

Splitters often don't output one pin per track so the streams list could be quite incomplete.
Subtitle render: Fix an off-by-one error when painting the overlay.

This error introduced in 3244957f4563575d22bf109de27a703558611b19 could cause artifacts to appear when using karaoke effects.

This commit fixes #2773.