Ignore the material update flag w/o vertex color.

Instead of using materialambient for all of them.

Fixes #3918, and seems to match tests with 3dstudio.
Fix typesafety (ScreenCoords not DrawingCoords.)
softgpu: Use region for debugger buffer previews.

It's fine if it resizes or etc., but this should generally be the
interesting area of vram to show, without any confusing noise on the
side/etc.
softgpu: Implement vertex preview.

And move ConvertMatrix4x3To4x4() into a common place since there were
differing implementations, which was only confusing.
softgpu: Fix preview region y1 in ge debugger.
Merge pull request #4937 from unknownbrackets/gpu-minor

Ignore material update when no vertex color, softgpu vertex preview