Issue #25 (2): Collision detection sometimes hangs.

Sometimes the expanding polytype algorithm takes a long time to
terminate, e.g. in situations like this:

 .-.
: +-:-+
 '-'  |
  |   |
  +---+

The issue here is that the EPA adds vertices very close to each other,
which makes the separation distance change very slowly. If this is the
case, consider the current separation distance as approximately correct
and continue.

gjk.lua

@@ -65,14 +65,17 @@ local function EPA(shape_a, shape_b, simplex)
 	end
 
 	-- the expanding polytype algorithm
+	local last_diff_dist = math.huge
 	while true do
 		local e = closest_edge(simplex)
 		local px,py = support(shape_a, shape_b, e.nx, e.ny)
 		local d = vector.dot(px,py, e.nx, e.ny)
 
-		if d - e.dist < 1e-6 then
+		local diff_dist = d - e.dist 
+		if diff_dist < 1e-6 or last_diff_dist - diff_dist < 1e-10 then
 			return -d*e.nx, -d*e.ny
 		end
+		last_diff_dist = diff_dist
 
 		-- simplex = {..., simplex[e.i-1], px, py, simplex[e.i]
 		table.insert(simplex, e.i, py)