Get stencil/depth reading for ge debugger.

Possibly works on NVIDIA only...
Fix a possibly wrong framebuffer pointer.
Add plumbing to show depth/stencil in ge debugger.
In ge debugger, ignore tex alpha based on texfunc.
Double click in ge dbg to toggle shrink-to-fit.
Make it possible to pan the texture while zoomed.
Merge pull request #4052 from unknownbrackets/debugger

GE debugger - fix depth/stencil, pan/zoom the texture