Make wxBitmap::Rescale() less horrible for commonly used icons

Make the results look somewhat better by using wxIMAGE_QUALITY_NEAREST
which preserves sharp horizontal and vertical edges in the images which
are common in the icons, with which this function is often used, instead
of blurring them as wxIMAGE_QUALITY_HIGH does.

This is also much (factor of ~40) faster, which shouldn't hurt neither.

src/common/bmpbase.cpp

@@ -70,8 +70,11 @@ void wxBitmapHelpers::Rescale(wxBitmap& bmp, const wxSize& sizeNeeded)
     wxCHECK_RET( sizeNeeded.IsFullySpecified(), wxS("New size must be given") );
 
 #if wxUSE_IMAGE
+    // Note that we use "nearest" rescale mode here to preserve sharp edges in
+    // the icons for which this function is often used. It's also consistent
+    // with what wxDC::DrawBitmap() does, i.e. the fallback method below.
     wxImage img = bmp.ConvertToImage();
-    img.Rescale(sizeNeeded.x, sizeNeeded.y, wxIMAGE_QUALITY_HIGH);
+    img.Rescale(sizeNeeded.x, sizeNeeded.y, wxIMAGE_QUALITY_NEAREST);
     bmp = wxBitmap(img);
 #else // !wxUSE_IMAGE
     // Fallback method of scaling the bitmap