misc: change image slice scale to 200% of style's specified

Want to circle back and find a better pattern for this as it enlarges the slice window (which doesn't clamp the preview window) and it unnecessarily upscales small images which bloats storage a bit. There's probably also something smarter to do with just saving "as large as possible" rather than strictly template size.

src/gui/value/image.cpp

@@ -41,7 +41,8 @@ void ImageValueEditor::sliceImage(const Image& image) {
   AlphaMask mask;
   style().mask.getNoCache(options,mask);
   // slice
-  ImageSliceWindow s(wxGetTopLevelParent(&editor()), image, style().getSize(), mask);
+  RealSize desiredSliceSize = RealSize(style().getSize().width * 2, style().getSize().height * 2);
+  ImageSliceWindow s(wxGetTopLevelParent(&editor()), image, desiredSliceSize, mask);
   // clicked ok?
   if (s.ShowModal() == wxID_OK) {
     // store the image into the set