Fudge the dpi into something that makes for more rational scaling to twips

gfx/src/gtk/nsDeviceContextGTK.cpp

@@ -64,12 +64,34 @@ NS_IMETHODIMP nsDeviceContextGTK::Init(nsNativeWidget aNativeWidget)
 
   mWidget = aNativeWidget;
 
-// this is used for something odd.  who knows
-  mTwipsToPixels = (((float)::gdk_screen_width()) /
-                    ((float)::gdk_screen_width_mm()) * 25.4) /
+  // Compute dpi of display
+  float screenWidth = float(::gdk_screen_width());
+  float screenWidthIn = float(::gdk_screen_width_mm()) / 25.4f;
+  nscoord dpi = nscoord(screenWidth / screenWidthIn);
+
+  // Now for some wacky heuristics. 
+  if (dpi < 84) dpi = 72;
+  else if (dpi < 108) dpi = 96;
+  else if (dpi < 132) dpi = 120;
+
+  mTwipsToPixels = float(dpi) / float(NSIntPointsToTwips(72));
+  mPixelsToTwips = 1.0f / mTwipsToPixels;
+
+#ifdef DEBUG
+  static PRBool once = PR_TRUE;
+  if (once) {
+    printf("GFX: dpi=%d t2p=%g p2t=%g\n", dpi, mTwipsToPixels, mPixelsToTwips);
+    once = PR_FALSE;
+  }
+#endif
+
+#if 0
+  mTwipsToPixels = ( ((float)::gdk_screen_width()) /
+                     ((float)::gdk_screen_width_mm()) * 25.4) /
 		     (float)NSIntPointsToTwips(72);
 
   mPixelsToTwips = 1.0f / mTwipsToPixels;
+#endif
 
   vis = gdk_rgb_get_visual();
   mDepth = vis->depth;