* Draw grid only over the Canvas (when in tiling mode)
* Replace some magic numbers with enums.
It's too easy to break something when adding something new in here. Should be a little harder now.
* Added Pixel Grid.
- Pixel grid is shown only when it's enabled and camera is zoomed close enough.
- Settings: pixel_grid_show_at_zoom (as a percentage because that's what's shown in the settings panel), pixel_grid_color. Default values might need changing.
- To distinguish between grid and pixel grid default settings for grid width, grid height are changed.
- Now both grid and pixel grid are drawn above (after) tile mode. Grid is drawn above (after) pixel grid.
