WGSL built-in similar to `gl_DrawID` (for Multi Draw Indirect)

[inodentry]

Is your feature request related to a problem? Please describe.

When using Multi Draw Indirect, there is no way for the shader to know what draw it is a part of.

GLSL in OpenGL provides the gl_DrawID built-in for this purpose. WGSL does not have an equivalent.

(AFAIK, the WebGPU spec does not have MDI at all? I am not sure. That would explain the lack of such builtin in WGSL.)

Describe the solution you'd like

Naga could add support for a draw_index built-in, similar to vertex_index and instance_index. Even though this is not part of the upstream WGSL spec.

Describe alternatives you've considered
Depending on the use case, it may be possible to work around this by using the instance IDs instead.

[jannik4]

I have also needed this for sometime and just used a fork. Currently it is just one line: jannik4@dce431a

(But of course I would have no problem if that would be supported here:))