PR bevyengine#17688 broke motion vector computation, and therefore motion blur,
because it enabled retention of `MeshInputUniform`s, and
`MeshInputUniform`s contain the indices of the previous frame's
transform and the previous frame's skinned mesh joint matrices. On frame
N, if a `MeshInputUniform` is retained on GPU from the previous frame,
the `previous_input_index` and `previous_skin_index` would refer to the
indices for frame N - 2, not the index for frame N - 1.

This patch fixes the problems. It solves these issues in two different
ways, one for transforms and one for skins:

1. To fix transforms, this patch supplies the *frame index* to the
   shader as part of the view uniforms, and specifies which frame index
   each mesh's previous transform refers to. So, in the situation
   described above, the frame index would be N, the previous frame index
   would be N - 1, and the `previous_input_frame_number` would be N - 2.
   The shader can now detect this situation and infer that the mesh has
   been retained, and can therefore conclude that the mesh's transform
   hasn't changed.

2. To fix skins, this patch replaces the explicit `previous_skin_index`
   with an invariant that the index of the joints for the current frame
   and the index of the joints for the previous frame are the same. This
   means that the `MeshInputUniform` never has to be updated even if the
   skin is animated. The downside is that we have to copy joint matrices
   from the previous frame's buffer to the current frame's buffer in
   `extract_skins`.

The rationale behind (2) is that we currently have no mechanism to
detect when joints that affect a skin have been updated, short of
comparing all the transforms and setting a flag for
`extract_meshes_for_gpu_building` to consume, which would regress
performance as we want `extract_skins` and
`extract_meshes_for_gpu_building` to be able to run in parallel.

To test this change, use `cargo run --example motion_blur`.

… change. (bevyengine#17688)

This patch fixes a bug whereby we're re-extracting every mesh every
frame. It's a regression from PR bevyengine#17413. The code in question has
actually been in the tree with this bug for quite a while; it's that
just the code didn't actually run unless the renderer considered the
previous view transforms necessary. Occlusion culling expanded the set
of circumstances under which Bevy computes the previous view transforms,
causing this bug to appear more often.

This patch fixes the issue by checking to see if the previous transform
of a mesh actually differs from the current transform before copying the
current transform to the previous transform.

PR #17688 broke motion vector computation, and therefore motion blur,
because it enabled retention of `MeshInputUniform`s, and
`MeshInputUniform`s contain the indices of the previous frame's
transform and the previous frame's skinned mesh joint matrices. On frame
N, if a `MeshInputUniform` is retained on GPU from the previous frame,
the `previous_input_index` and `previous_skin_index` would refer to the
indices for frame N - 2, not the index for frame N - 1.

This patch fixes the problems. It solves these issues in two different
ways, one for transforms and one for skins:

1. To fix transforms, this patch supplies the *frame index* to the
shader as part of the view uniforms, and specifies which frame index
each mesh's previous transform refers to. So, in the situation described
above, the frame index would be N, the previous frame index would be N -
1, and the `previous_input_frame_number` would be N - 2. The shader can
now detect this situation and infer that the mesh has been retained, and
can therefore conclude that the mesh's transform hasn't changed.

2. To fix skins, this patch replaces the explicit `previous_skin_index`
with an invariant that the index of the joints for the current frame and
the index of the joints for the previous frame are the same. This means
that the `MeshInputUniform` never has to be updated even if the skin is
animated. The downside is that we have to copy joint matrices from the
previous frame's buffer to the current frame's buffer in
`extract_skins`.

The rationale behind (2) is that we currently have no mechanism to
detect when joints that affect a skin have been updated, short of
comparing all the transforms and setting a flag for
`extract_meshes_for_gpu_building` to consume, which would regress
performance as we want `extract_skins` and
`extract_meshes_for_gpu_building` to be able to run in parallel.

To test this change, use `cargo run --example motion_blur`.