Fix compilation with GCC

- GCC is pickier than Visual C++. GCC requires that structs with constructors, etc that are
  used in a union must be defined outside the union. VC++ lets you do it inline.

src/r_things.cpp

@@ -678,7 +678,7 @@ void R_DrawVisVoxel(vissprite_t *spr, int minslabz, int maxslabz, short *cliptop
 	}
 
 	// Render the voxel, either directly to the screen or offscreen.
-	R_DrawVoxel(spr->vpos, spr->vang, spr->gpos, spr->angle,
+	R_DrawVoxel(spr->pa.vpos, spr->pa.vang, spr->gpos, spr->angle,
 		spr->xscale, FLOAT2FIXED(spr->yscale), spr->voxel, spr->Style.colormap, cliptop, clipbot,
 		minslabz, maxslabz, flags);
 
@@ -1033,8 +1033,8 @@ void R_ProjectSprite (AActor *thing, int fakeside, F3DFloor *fakefloor, F3DFloor
 			vis->angle -= ang.BAMs();
 		}
 
-		vis->vpos = { (float)ViewPos.X, (float)ViewPos.Y, (float)ViewPos.Z };
-		vis->vang = FAngle((float)ViewAngle.Degrees);
+		vis->pa.vpos = { (float)ViewPos.X, (float)ViewPos.Y, (float)ViewPos.Z };
+		vis->pa.vang = FAngle((float)ViewAngle.Degrees);
 	}
 
 	// killough 3/27/98: save sector for special clipping later
@@ -1292,7 +1292,7 @@ void R_DrawPSprite (pspdef_t* psp, int pspnum, AActor *owner, double sx, double
 
 	if (avisp[0] == NULL)
 	{
-		for (int i = 0; i < countof(avis); ++i)
+		for (unsigned i = 0; i < countof(avis); ++i)
 		{
 			avisp[i] = &avis[i];
 		}

src/r_things.h

@@ -31,6 +31,12 @@
 
 struct vissprite_t
 {
+	struct posang
+	{
+		FVector3 vpos;			// view origin
+		FAngle vang;			// view angle
+	};
+
 	short			x1, x2;
 	FVector3		gpos;			// origin in world coordinates
 	union
@@ -61,16 +67,9 @@ struct vissprite_t
 			fixed_t	xiscale;		// negative if flipped
 		};
 		// Used by wall sprites
-		struct
-		{
 		FWallCoords wallc;
-		};
 		// Used by voxels
-		struct
-		{
-			FVector3 vpos;			// view origin
-			FAngle vang;			// view angle
-		};
+		posang pa;
 	};
 	sector_t		*heightsec;		// killough 3/27/98: height sector for underwater/fake ceiling
 	sector_t		*sector;		// [RH] sector this sprite is in