Validation fixes for dual-source blending, depth bias, and frag_depth

[andyleiserson]

Several validation fixes. The only reason the dual-source blending fix is related is that both it and the frag_depth fix add fields to ShaderStageForValidation::Fragment that @ErichDonGubler added recently.

• If the pipeline uses dual-source blending, then the shader must configure appropriate I/O, but it is not an error to use a shader that defines the I/O in a pipeline that only uses one blend source. Fixes dual source blending is forced if enabled (wgpu being overly strict?) #8592.
• Depth bias is only supported with triangle topologies. Fixes Point and line primitives must not have depth bias #6071.
• If the shader outputs frag_depth, then the pipeline must have a depth attachment. (No bug.)

This change will conflict with #8840, which needs more discussion, so I expect #8840 will be rebased on this one. Whichever lands second should enable the entire depth_stencil_state suite.

Testing
Enables relevant CTS tests.

Squash or Rebase? Rebase

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• Add CHANGELOG.md entries.