Change bevy_pbr::morph shader functions to use descriptor index

[komadori]

Objective

The shader functions bevy_pbr::morph take an instance_index as a parameter and use it solely to look up the morph_descriptor_index in the mesh array. This prevents these functions from being used in an environment without instance indices and the mesh array available.

This is closely related to #24083 and porting bevy_mod_outline to 0.19-dev. I have put them in separate PRs because I consider #24083 a prerequisite to supporting morphed meshes without significant duplication both of code and work at runtime, and it is a very trivial change. I am in favour of this PR also, but if it is not merged for 0.19 then it is tractable to duplicate Bevy's morph shader functions inside bevy_mod_outline.

Solution

• Change the shader functions in bevy_pbr::morph take the descriptor index as a parameter instead of the instance index and to not bind the mesh array.

Testing

• Ran the morph_targets and many_morph_targets examples.