When PROJ_ROUND()ing e.g. 3.8, it ends up at 4, then we added the 0.5
offset to draw a nice cairo line in the middle of the pixel,
effectively drawing a line that's meant to be at 3.8 at 4.5.  Instead,
we now use floor(x)+0.5 now which snaps the above example to 3.5.

Also, calculate arcs like we calculate rectangles (transform the arc's
bounding box and pixel-align that, then recalculate the center), so
arcs properly align with rectangles.