test-clip: Replace the ellipse with a rotated rectangle

test-clip: Replace the ellipse with a rotated rectangle

With test-clip it's possible to draw three different shapes depending
on what mouse button is used: a rectangle, an ellipse or a path
containing multiple shapes. However the ellipse is also a path so it
doesn't really test anything extra from the third option. This
replaces the ellipse with a rectangle that is first rotated by the
modelview matrix. The rotated matrix won't be able to use the scissor
so it can be used to test stencil and clip plane clipping.