Replace instance index vertex output with mesh flags

[JMS55]

Objective

• Fragment shaders are currently coupled to our specific vertex shaders, as the mesh flags are read from the mesh bindings in the fragment shader. This prevents reusing the fragment shaders in alternative renderers that do not use the mesh bindings, such as Meshlet rendering (initial feature) #10164.

Solution

• Vertex shaders now return the mesh flags directly, replacing the previous instance_index output.

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Changelog

• Vertex shaders now output mesh flags directly instead of the instance index.
  • The VERTEX_OUTPUT_INSTANCE_INDEX shader def has been replaced by VERTEX_OUTPUT_MESH_FLAGS.
  • VertexOutput instance_index fields have been replaced by the new mesh_flags field.

Migration Guide

• Fragment shaders using VertexOutput::instance_index to fetch the mesh flags should now use VertexOutput::mesh_flags directly.
• Vertex shaders that were outputting VertexOutput::instance_index for use with fragment shaders that read the mesh flags via the instance index should now read the mesh flags and output the flags themselves via VertexOutput::mesh_flags.

[JMS55]

@Elabajaba brought up that we should probably profile with NSight to confirm this doesn't impact performance.

[superdump]

One other reason to pass the instance index through was for custom fragment shaders to be able to use it to index into things.

[JMS55]

I no longer need this for meshlets, so the only reason to change this would be perf.