Closes #1045

While debugging I cleaned a few things up.

• The actual fix is adding stub padding variables to the influences buffer.
• Fix a case where a template var (use_morph_targets) was meant to store a bool, but ended up storing a tuple of arrays, which then participate to the shader's hash.
• The morph count is the minimum of the lengths of the different morph arrays, similar to how Python's zip iterates up to the smallest size.
• Use an explicit size for the influences buffer.
• Use the actual max texture size.

cc @panxinmiao