Simplify clip_transformed() by replacing its context parameter with a
clipping box parameter. The context struct is still used internally to
pass data around.
Since clip_transformed() doesn't take a context anymore, the clipping
boxes are now declared per test and stored along with the other vertex
data. That prepares the ground to add new tests using different boxes.
Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>