EMI: Fix vertex skinning

This allows the vertices of a model to follow multiple joints of the skeleton, respecting blend weights as defined in the model file. Previously a vertex only followed one of the associated joints and ignored the others. This caused some animations to look bad. This fixes Mr. Cheese's leg, for example: 

Old:
![residualvm 2014-06-10 15-12-48-86](https://cloud.githubusercontent.com/assets/757718/3294902/d0d8880e-f5b2-11e3-8172-8c91838da6e0.png)

New (ignore the FPS!):
![residualvm 2014-06-17 03-03-41-40](https://cloud.githubusercontent.com/assets/757718/3294912/210c1868-f5b3-11e3-809f-a4df6d01a59a.png)

This also improves Charles L. Charles' animation when he first appears after completing the catapult puzzle.