basegfx: accelerate Trapezoid subdivision by avoiding allocations.

Allocate on the stack where possible, then more conservatively on
the heap. Unit test Trapezoid subdivision for good measure.

Change-Id: I83e63fd55fddcad149bb5279865670ebb18e0ab1
fdo#73021 - avoid call crash on spell-check + unit test.

Change-Id: I3588ef45617bda440f970e54274cc0985b7baed5