feature : zoom

- Added MinimapController that displays a frame on the animated preview
  when zoomed in
- Added bounds for the offset to make sure it doesn't go crazy
- Added new utility Math.js with a minmax function
- TODO : the minimap controller has a lot of dependencies, see if could be
  cleaned up
- TODO : DrawingController knows the size of the picture it has to render
  only indirectly, which makes it hard in some cases (such as boundary
  checking performed during setOffset)