some work on extreme/degenerate shit in GPU

* clip against Z then Y then X. apparently, fixes #310. I had also observed hints that the hardware does it this way.
* truncate W to 24 bits before viewport transform.
* mark any polygons that have a W=0 at that point as degenerate. do not render.

src/GPU3D.h

@@ -54,6 +54,8 @@ typedef struct
     u32 TexParam;
     u32 TexPalette;
 
+    bool Degenerate;
+
     bool FacingView;
     bool Translucent;