js: Distinguish manual resizes from device pixel ratio changes

This adds a new callback, rescale_puzzle(), that's called when the
device pixel ratio changes.  This means that resize_puzzle() can safely
set the nominal canvas size, which means that manual resizing of the
puzzle now sticks.

Still missing: paying attention to the device pixel ratio when choosing
the initial (or reset) size.

cmake/platforms/emscripten.cmake

@@ -26,6 +26,8 @@ set(emcc_export_list
   # Callbacks when the resizing controls are used
   _resize_puzzle
   _restore_puzzle_size
+  # Callback when device pixel ratio changes
+  _rescale_puzzle
   # Main program, run at initialisation time
   _main)