fit-model-ceres assert/crash when using with a model that doesn't have exactly 6 blendshapes

[patrikhuber]

I should test fit-model-ceres with a model that doesn't have exactly 6 blendshapes (e.g. the 4DFM), as the example has the number 6 hard-coded. According to a report, this will hit an assert somewhere in sample_to_mesh with an Eigen multiplication matrix dimension mismatch.

It would be good if we could update the Ceres example so it at least works with other models without hitting any asserts/errors.