Centroid selection for variable rate shading

A technique for determining the centroid for fragments generated using variable rate shading. Because the barycentric interpolation used to determine texture coordinates for pixels is based on the premise that the point being interpolated is within the triangle, centroids that are outside of the triangle can produce undesirable visual artifacts. Another concern, however, is that the further the centroid is from the center of a pixel, the less accurate quad-based pixel derivatives become for attributes of that pixel. To address these concerns, the position of the sample that is both covered by the triangle and the closest to the center of the pixel, out of all covered samples of the pixel, is used as the centroid for a partially covered pixel. For a fully covered pixel (all samples in a pixel are covered by a triangle), the center of that pixel is used as the centroid.