ddraw/tests: Don't destroy the background material until the viewport is done with it in test_material() (Valgrind).

Most of it was done by Andrew Eikum.

While at it, use D3D-like uniforms for the light parameters and
(consequently) implement range and proper D3D spotlights support.

It's only an implementation detail that the direction needs to be passed
to the fixed function GL pipeline as GL_POSITION.

It's a direction so we don't want to get translations applied to it.
The variable is only used internally.