fix(webgpu): Insert fragment constants into fragment descriptor

[DerSchmale]

Description
In create_render_pipeline, the frag.compilation_options.constants were mistakenly inserted into mapped_vertex_state instead of mapped_fragment_desc, causing override constants to fail in the fragment shader.

Testing
A shader was used to conditionally sample a texture to use as color output, otherwise output white. Did not work before, but does now.

Squash or Rebase?

Not applicable.