Refactor heatmap for quad overdraw/triangle size to be applied late

* Instead of baking these into the overlay texture and trying to decode them
  afterwards, we instead write a grayscale 16F value into the overlay texture,
  and add a special decode display mode that will use the heatmap and bucketing
  provided.
* This means that saving these overlay textures now saves grayscale. When saving
  to an 8-bit format, we remap to 0-255 so that greater than 1.0 values are
  mapped lower.