Fix motion blur of instanced skinned geometry with animated parent matrix

[cpichard]

Changes proposed in this pull request

• We interpolate transform matrices using the max number of keys between the parent and the current evaluated matrix. Before this patch, only the number of keys of the current matrix was used which could cause issue when evaluating prototypes connected to animated instances.
• If the prim is in a prototype, we can't know if its parent instance is animated, so we return true in the function checking if the transform might be varying.
• We add a test extracted from the scene provided in 1837, this test is reduced to a single geo to make it leaner.

Issues fixed in this pull request
Fixes #1837