Add HC:addShape(shape).

Shape must have at least two functions:
- shape:bbox()
  must return an axis aligned bounding box of the shape

- shape:collidesWith(other)
  must return false, ... if the two shapes do not collide
  must return true, sx, sy if the two shapes collide, where sx,sy is the
                           separation vector from from shape to other.

Convex shapes can use the supplied GJK algorithm. In that case the shape
must implement shape:support(dx,dy) which returns the point of the shape
that lies furthest in the direction of dx,dy.

init.lua

@@ -86,13 +86,14 @@ function HC:setCallbacks(collide, stop)
 	return self
 end
 
-local function new_shape(self, shape)
-	local x1,y1,x2,y2 = shape:bbox()
+function HC:addShape(shape)
+	assert(shape.bbox and shape.collidesWith,
+		"Cannot add custom shape: Incompatible shape.")
 
 	self._current_shape_id = self._current_shape_id + 1
 	self._active_shapes[self._current_shape_id] = shape
 	self._shape_ids[shape] = self._current_shape_id
-	self._hash:insert(shape, x1,y1, x2,y2)
+	self._hash:insert(shape, shape:bbox())
 	shape._groups = {}
 
 	local hash = self._hash
@@ -129,7 +130,7 @@ function HC:activeShapes()
 end
 
 function HC:addPolygon(...)
-	return new_shape(self, newPolygonShape(...))
+	return self:addShape(newPolygonShape(...))
 end
 
 function HC:addRectangle(x,y,w,h)
@@ -137,11 +138,11 @@ function HC:addRectangle(x,y,w,h)
 end
 
 function HC:addCircle(cx, cy, radius)
-	return new_shape(self, newCircleShape(cx,cy, radius))
+	return self:addShape(newCircleShape(cx,cy, radius))
 end
 
 function HC:addPoint(x,y)
-	return new_shape(self, newPointShape(x,y))
+	return self:addShape(newPointShape(x,y))
 end
 
 function HC:share_group(shape, other)
@@ -165,7 +166,7 @@ function HC:update(dt)
 	-- collect colliding shapes
 	for shape in self:activeShapes() do
 		tested[shape] = {}
-		for other in hash:rangeIter(shape:bbox()) do
+		for other in self._hash:rangeIter(shape:bbox()) do
 			if not may_skip_test(shape, other) then
 				local collide, sx,sy = shape:collidesWith(other)
 				if collide then