Fix UTF grid point rendering
In 256x256 image tiles, we render points as circles with 8-pixel radius and need to scale appropriately for smaller tiles.
In the future we will use markers for points rather than fixed-radius circles.
Fix marker rendering
Fix references to the radius and line-width.
Honor marker-fill-opacity and marker-line-opacity.
Render markers in the UTF grid.
Fix parsing for opacity values of 0