work on optimizations
big optimization : only draw blocks and edges that are on the screen zone (with help of aabb from box2D)
optimization : start of double buffer (second canvas)
double buffering optimization : it works but I still have to refresh the buffer canvas and deal with the zoom when I copy the buffer into the main canvas
adaptation of code for buffer of any size
double buffering : the buffer is copied at the right position on the canvas (even with the zoom)
really nice use of double buffering. The buffer redraw automatcally at the moto position when the view of the main canvas go outside of what's in the buffer
double buffer refactoring
refactoring
buffer scale optimization
easy full screen
better physics (filter for blocks and edges)
strange diff
big optimization : lines instead of triangulation of polygons and each block is now an object with its own fixtures. Very very fast
Merge branch 'master' into replays

Conflicts:
	bin/xmoto.js
	index.html
	src/level.coffee
replaying : minor improvements

temporarly move the camera to the biker
display entity in ugly mode

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>