Fix snapshots on retina displays.

src/common/FrameBufferSDL2.hxx

@@ -137,12 +137,12 @@ class FrameBufferSDL2 : public FrameBuffer
     /**
       Transform from window to renderer coordinates, x direction
      */
-    int scaleX(int x) const { return (x * myRenderW) / myWindowW; }
+    int scaleX(int x) const override { return (x * myRenderW) / myWindowW; }
 
     /**
       Transform from window to renderer coordinates, y direction
      */
-    int scaleY(int y) const { return (y * myRenderH) / myWindowH; }
+    int scaleY(int y) const override { return (y * myRenderH) / myWindowH; }
 
   protected:
     //////////////////////////////////////////////////////////////////////

src/common/PNGLibrary.cxx

@@ -29,6 +29,7 @@
 #include "TIASurface.hxx"
 #include "Version.hxx"
 #include "PNGLibrary.hxx"
+#include "Rect.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 PNGLibrary::PNGLibrary(OSystem& osystem)
@@ -129,7 +130,13 @@ void PNGLibrary::saveImage(const string& filename, const VariantList& comments)
     throw runtime_error("ERROR: Couldn't create snapshot file");
 
   const FrameBuffer& fb = myOSystem.frameBuffer();
-  const Common::Rect& rect = fb.imageRect();
+
+  const Common::Rect& rectUnscaled = fb.imageRect();
+  const Common::Rect rect(
+    Common::Point(fb.scaleX(rectUnscaled.x()), fb.scaleY(rectUnscaled.y())),
+    fb.scaleX(rectUnscaled.w()), fb.scaleY(rectUnscaled.h())
+  );
+
   png_uint_32 width = rect.w(), height = rect.h();
 
   // Get framebuffer pixel data (we get ABGR format)

src/emucore/FrameBuffer.hxx

@@ -381,6 +381,16 @@ class FrameBuffer
     */
     virtual void clear() = 0;
 
+    /**
+      Transform from window to renderer coordinates, x direction
+     */
+    virtual int scaleX(int x) const { return x; }
+
+    /**
+      Transform from window to renderer coordinates, y direction
+     */
+    virtual int scaleY(int y) const { return y; }
+
   protected:
     /**
       This method is called to query and initialize the video hardware