Subtitle queue: Fix sluggish rendering of animated subtitles.

After 8d494fcd586c1bc75f8493d2fa68d5784394aeb3, the queue started using the time per frame to avoid rendering useless subpicture. However the time per frame we get from the filter chain can be an estimate which induces slight mispredictions in the start and end times of each frames. This can sometimes cause issues when rendering animated subtitles because the wrong subtitle is picked up.

This commit fixes those issues by using some more or less hackish work-arounds until this problem can be fixed in a cleaner way.

Ref: #4831.
Subpic: Fix the segment start in rare cases.

0 is a perfectly valid time for segment start so use something else to denote invalid time.

Fixes #4831.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...