Ignore pivot channels in animations.
Fixed Song.PlayPosition that is invalid on Android after a Play-Pause cycle
Merge commit '495761c' into ModelBacklog
Fixed bad keyframe continuity.
XNA appears to default this to null, not string.Empty
More model refinements.

(Cherry picked minus duplicated effort in ModelProcessor)
Merge pull request #3075 from SickheadGames/ModelBacklog

Misc model pipeline fixes
Merge pull request #3074 from SongArc/feature/android_playpause_fix

Fixed Song.PlayPosition on Android
Fix for save as JPG for ios and android.
Merge branch 'develop' of http://github.com/mono/MonoGame into develop