VIDEO: Check validity of granulepos in TheoraVideoTrack::decodePacket - #7427

A fix was introduced in 43c5d3e, which adds handling for dropped frames in theora decoding. However, this didn't handle the case where the granule position for the current packet is invalid.

These changes introduce a check on the granule position of the current packet. If the granule position is valid, proceed to properly calculate the frame number and the next frame start time from the granule position. If the granule position is not valid, use best estimation of for these values.

These changes also refactor to combine the checks for the two cases where the granule position is passed to theora functions. The documentation for both of these functions states that they will return -1 in the case that the provided granule position is negative.

Background

I was trying to play Syberia on ScummVM v2026.2.0, and found it unplayable due to the choppy video playback in the main menu and intro video, which also seemingly affects the UI in the game with high latency.

I found v2026.1.0 worked OK, so did a git bisect to locate the commit which introduced the problem for me as 43c5d3e.

Through some debugging I could see that oggPacket.granulepos was often -1, which led to _curFrame being reset to -1 frequently throughout video playback. It seems that the code below for calculating _nextFrameStartTime was handling this case, so I applied a similar fix to calling th_granule_frame, but switching the condition to be on oggPacket.granulepos rather than calling the function and checking if the result is invalid. I think this is a reasonable change, having read the documentation for th_granule_frame and th_granule_time, but I can change the approach to check the result of the function call rather than its input if that is preferable.