0.12 Migration: Reduce the size of MeshUniform section shader code is broken

[rparrett]

affine_to_square isn't even in mesh_functions anymore. It was moved in bevyengine/bevy#9597.

But also, it seems that you want to use mesh_functions::get_model_matrix for this now instead, due to it handling the details of some workaround for some DX12 bug.

Relevant bit of the migration guide:

bevy-website/content/learn/migration-guides/0.11-0.12/_index.md

Lines 923 to 933 in bf681ad

	`MeshUniform` now stores its current and previous model transforms as 4x3 matrices. Helper functions were added to bevy_pbr::mesh_functions to unpack the data.
	```rust
	// 0.11
	var model = mesh[instance_index].model;
	// 0.12
	#import bevy_pbr::mesh_functions::affine_to_square
	var model = affine_to_square(mesh[instance_index].model);
	```

Relevant text from Discord:

robswain — Today at 7:11 AM
yes. i'd recommend doing what mesh.wgsl does (if i'm remembering the correct file)

robswain — Today at 7:12 AM
https://github.com/bevyengine/bevy/blob/4852fc7578b7a55db4c05a6fb9f8ebdd290571e1/crates/bevy_pbr/src/render/mesh.wgsl#L46
https://github.com/bevyengine/bevy/blob/4852fc7578b7a55db4c05a6fb9f8ebdd290571e1/crates/bevy_pbr/src/render/mesh_functions.wgsl#L15

robswain — Today at 7:27 AM
and if you want to see a bit more about what the problem with instance index is, there's a comment here: https://github.com/bevyengine/bevy/blob/main/crates/bevy_render/src/instance_index.wgsl
get_model_matrix was to abstract that away and give people something simpler to use

mesh[vertex.instance_index].model seems to show up elsewhere in the migration guide as well...