This fixes a bug in FrameBuffer.ellipse where it goes into an infinite loop
if both radii are 0.
This fixes the bug with a simple pre-check to see if both radii are 0, and
in that case sets a single pixel at the center. This is consistent with the
behaviour of the method when called with just one of the radii set to 0,
where it will draw a horizontal or vertical line of 1 pixel width.
The pixel is set with setpixel_checked so it should handle out-of-bounds
drawing correctly.
This fix also includes three new tests: one for the default behaviour, one
for drawing out-of-bounds, and one for when the sector mask is 0.
Fixes issue #16053.
Signed-off-by: Corran Webster <cwebster@unital.dev>
