Move sky plane-drawing code from R_DrawPlanes to a new function called by R_DrawSinglePlane

This potentially allows FOFs and polyobjects to display sky flats on them properly, unless skyboxes are involved in which case they'd fail either way

I also updated or added comments to some places of the sky drawing code and related where useful

src/r_plane.c

@@ -694,10 +694,10 @@ void R_MakeSpans(INT32 x, INT32 t1, INT32 b1, INT32 t2, INT32 b2)
 void R_DrawPlanes(void)
 {
 	visplane_t *pl;
-	INT32 x;
-	INT32 angle;
 	INT32 i;
 
+	// Note: are these two lines really needed?
+	// R_DrawSinglePlane and R_DrawSkyPlane do span/column drawer resets themselves anyway
 	spanfunc = basespanfunc;
 	wallcolfunc = walldrawerfunc;
 
@@ -705,22 +705,53 @@ void R_DrawPlanes(void)
 	{
 		for (pl = visplanes[i]; pl; pl = pl->next)
 		{
-			// sky flat
+			if (pl->ffloor != NULL
-			if (pl->picnum == skyflatnum)
+#ifdef POLYOBJECTS_PLANES
-			{
+			|| pl->polyobj != NULL
+#endif
+			)
+				continue;
+
+			R_DrawSinglePlane(pl);
+		}
+	}
+#ifndef NOWATER
+	waterofs = (leveltime & 1)*16384;
+	wtofs = leveltime * 140;
+#endif
+}
+
+// R_DrawSkyPlane
+//
+// Draws the sky within the plane's top/bottom bounds
+// Note: this uses column drawers instead of span drawers, since the sky is always a texture
+//
+static void R_DrawSkyPlane(visplane_t *pl)
+{
+	INT32 x;
+	INT32 angle;
+
+	// If we're not supposed to draw the sky (e.g. for skyboxes), don't do anything!
+	// This probably utterly ruins sky rendering for FOFs and polyobjects, unfortunately
 	if (!viewsky)
 	{
+		// Mark that the sky was visible here for next tic
+		// (note: this is a hack and it sometimes can cause HOMs to appear for a tic IIRC)
 		skyVisible = true;
-					continue;
+		return;
 	}
 
+	// Reset column drawer function (note: couldn't we just call walldrawerfunc directly?)
+	// (that is, unless we'll need to switch drawers in future for some reason)
+	wallcolfunc = walldrawerfunc;
+
 	// use correct aspect ratio scale
 	dc_iscale = skyscale;
 
 	// Sky is always drawn full bright,
 	//  i.e. colormaps[0] is used.
 	// Because of this hack, sky is not affected
-				//  by INVUL inverse mapping.
+	//  by sector colormaps (INVUL inverse mapping is not implemented in SRB2 so is irrelevant).
 	dc_colormap = colormaps;
 	dc_texturemid = skytexturemid;
 	dc_texheight = textureheight[skytexture]
@@ -741,23 +772,6 @@ void R_DrawPlanes(void)
 			wallcolfunc();
 		}
 	}
-				continue;
-			}
-
-			if (pl->ffloor != NULL
-#ifdef POLYOBJECTS_PLANES
-			|| pl->polyobj != NULL
-#endif
-			)
-				continue;
-
-			R_DrawSinglePlane(pl);
-		}
-	}
-#ifndef NOWATER
-	waterofs = (leveltime & 1)*16384;
-	wtofs = leveltime * 140;
-#endif
 }
 
 void R_DrawSinglePlane(visplane_t *pl)
@@ -771,6 +785,13 @@ void R_DrawSinglePlane(visplane_t *pl)
 	if (!(pl->minx <= pl->maxx))
 		return;
 
+	// sky flat
+	if (pl->picnum == skyflatnum)
+	{
+		R_DrawSkyPlane(pl);
+		return;
+	}
+
 #ifndef NOWATER
 	itswater = false;
 #endif