Fix overlapping attributes when using dualSrcBlend feature

[Fuzzyzilla]

1. Update documentation to reflect any user-facing changes - in this repository.
   N/A, it was an internal bug

2. Make sure that the changes are covered by unit-tests.
   Looks like shader module as a whole is missing unit tests :<

3. Run cargo fmt on the changes.

4. Please put changelog entries in the description of this Pull Request
   if knowledge of this change could be valuable to users. No need to put the
   entries to the changelog directly, they will be transferred to the changelog
   file by maintainers right after the Pull Request merge.

   Please remove any items from the template below that are not applicable.

5. Describe in common words what is the purpose of this change, related
   Github Issues, and highlight important implementation aspects.

This change allows for fragment shaders to make use of the dualSrcBlend feature of Vulkan 1.0, by assigning an index layout to color outputs. In vulkano 0.33.0, this causes a compile time error, and in the current master branch it causes a runtime error, due to detection of overlapping interface variables not taking index into account. This fixes that :>

Changelog:

### Bugs fixed
- fragment shaders cannot use `dual_src_blend` device feature due to interface errors

[Rua]

The shader interface handling is due an overhaul anyway, to solve issues like #1738. But this is useful in the meantime, thanks!