polygons: Tag circular contours so they can be special-cased when drawing.

polygons: Tag circular contours so they can be special-cased when drawing.

Tagging circular contours allows GUIs (if they wish) to draw circular
holes in polygons more accurately, and potentially faster - depending
on whether they use mask based rendering or rely on the dicer.

When zoomed far out (and using a mask based scheme), the drawing routine
can use a lower vertex count approximation to the contour - leading to
rendering speed improvements. This is used to reasonable effect in the
experimental OpenGL branch.

Modify hid/common/draw_helpers.c to use these tags when thin-drawing
polygon contours with common_thindraw_pcb_polygon(). This allows the
GUI to change the level of detail rendered with zoom, and serves to
test this feature.

DRC checking and output are still done using the fixed resolution
approximation present in the polygon contour, so in this respect -
it makes rendering somewhat unfaithful to the final output.