Reject trapezoids where top (botttom) is above (below) the edges

Reject trapezoids where top (botttom) is above (below) the edges

When a trapezoid has a top/bottom that is above/below the left/right
edges, degenerate trapezoids become possible. For example the edge
could be very short and close to horizontal. If the bottom edge is far
below the bottom point of such a short edge, the result is that the
lower right corner of the trapezoid will be extremely far to the left.

This kind of trapezoid causes overflows in the rasterization code, so
change pixman_trapezoid_valid() to reject them.