make bevy_mesh optional for bevy_animate

[atlv24]

Objective

• make bevy_mesh optional for bevy_animate as mentioned in rename animation feature to bevy_animation #20714 (comment)

Solution

• copy paste the morph weights stuff out into a module, gate it, then add the optional dep

[greeble-dev]

I'm not sure M-Needs-Migration-Guide applies - change is backwards compatible with use bevy::animation::animation_curves::WeightsCurve and preludes.

[atlv24]

Migration guide is indeed not needed here.

[mockersf]

Shouldn't there be a morph_weights feature that enables bevy_mesh? It feels a bit backward to do it the other way.

[alice-i-cecile]

Shouldn't there be a morph_weights feature that enables bevy_mesh? It feels a bit backward to do it the other way.

This feels clearer to me too.

[atlv24]

I think that's an orthogonal request: bevy_mesh does not have a morph_weights feature. If it did, it would make sense to gate this on bevy_mesh/morph_weights, but right now this is about bevy_animation supporting bevy_mesh. The specific part of bevy_mesh it supports is morph weights, but this could just as easily be gating skinning code or other things.

[atlv24]

I'd be in favor of adding a morph_weights feature to bevy_mesh which gates it there and also in bevy_animate, anyhow. I just dont think its relevant to this PR