Change bevy_pbr::morph shader functions to use descriptor index (#24085)

# 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.

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Co-authored-by: Greeble <166992735+greeble-dev@users.noreply.github.com>
Co-authored-by: Carter Anderson <mcanders1@gmail.com>