This allows you to use just some of the frames of an animation
which has multiple facings.
For example, if you have an animation which has 8 facings, and 6 frames
of animation per facing, but you only want to show 3 of them, you can now do:
Facings: 8
Length: 3 # the number of frames in each facing to actually use
Stride: 6 # the number of frames per facing present in the SHP
If Stride is not specified, it defaults to Length, which is the old behavior.
It is an error to specify a stride less than the length -- this would produce
overlapping facings, or weirder.
