Native bindings to the NV_primitive_shading_rate extension.

This extension builds on top of the NV_shading_rate_image extension to provide OpenGL API support for using a per-primitive shading rate value to control the computation of the rate used to process each fragment.

In the NV_shading_rate_image extension, the shading rate for each fragment produced by a primitive is determined by looking up a texel in the bound shading rate image and using that value as an index into a shading rate palette. That extension provides a separate shading rate image lookup enable and palette for each viewport. When a primitive is rasterized, the implementation uses the enable and palette associated with the primitive's viewport to determine the shading rate.

This extension decouples the shading rate image enables and palettes from viewports. The number of enables/palettes now comes from the implementation-dependent constant SHADING_RATE_IMAGE_PALETTE_COUNT_NV. When SHADING_RATE_IMAGE_PER_PRIMITIVE_NV (added here) is enabled, the value of the new gl_ShadingRateNV built-in output is used to select an enable and palette to determine the shading rate. Otherwise, the viewport number for the primitive is used, as in NV_shading_rate_image.

Requires OpenGL 4.5 and NV_shading_rate_image.