Support multiple frame const animations

[ScanMountGoat]

The const types only require specifying a single frame value. The animation's final frame index determines the actual length.

• [x] add a frame count field to AnimData
• [x] return an error if a track has more frames than the specified frame count
• [x] zero length animations?

[ScanMountGoat]

Zero length animations cause visual issues in game but still render, so it might not be worth making this an error condition.