Fixed #2696: Sequences: Allow stride to be specified.

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.