Validate shader location clashes

[emilk]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Description
Validate that no two attributes are assigned to the same shader location.

"Each attrib in the union of all GPUVertexAttribute across descriptor.buffers has a distinct attrib.shaderLocation value." (https://www.w3.org/TR/webgpu/#abstract-opdef-validating-gpuvertexstate)

(Thanks to @Wumpf for finding that quote, and for guiding me to making this PR)

Testing
cargo nextest run --no-fail-fast works the same as on master, which means one failure:

        FAIL [   0.064s]                  wgpu::wgpu-tests zero_init_texture_after_discard::discarding_depth_target_resets_texture_init_state_check_visible_on_copy_in_same_encoder

--- STDOUT:                               wgpu::wgpu-tests zero_init_texture_after_discard::discarding_depth_target_resets_texture_init_state_check_visible_on_copy_in_same_encoder ---

running 1 test

--- STDERR:                               wgpu::wgpu-tests zero_init_texture_after_discard::discarding_depth_target_resets_texture_init_state_check_visible_on_copy_in_same_encoder ---
-[AGXG13XFamilyCommandBuffer renderCommandEncoderWithDescriptor:]:380: failed assertion `A command encoder is already encoding to this command buffer'

[teoxoy]

LGTM, thanks for the PR!

[teoxoy]

Could you open an issue about the unrelated test failure and include some info about your hardware/setup?

[cwfitzgerald]

Unfortunately, missed a typo here (also added a few words while we're at it):

Suggested change

	#[error("Two or more attributes were assigned to the same shader lcoation {0}")]
	#[error("Two or more vertex attributes were assigned to the same location in the shader: {0}")]

[emilk]

I'll make a PR with the typo fixes

[emilk]

Could you open an issue about the unrelated test failure and include some info about your hardware/setup?

#3614