Consider reporting max active morph targets

WebGL implementations are limited to 16 attributes, and may only allocate a subset of those to morph targets. Per the spec:

> A conformant client implementation must support at least eight morphed attributes.

It would be helpful if the validator could report the maximum number of simultaneously active morph targets in a model, and report a warning if that is greater than 8. Not sure if that's a reasonable request, scanning through all possible combinations of time and animation states would clearly be difficult. Maybe just use the maximum number of active morph targets used in any single animation, and ignore implications of multiple animations being played simultaneously?

The issue names no files or tests. Start by locating the validator's morph-target and animation validation entry points, then decide how to measure the maximum active targets under the proposed single-animation scope. Done means reporting that maximum and warning when it exceeds 8, with tests covering the chosen behavior.

Written by the indexing model from the issue text.