Work around math.acos(-1) returning NaN on LuaJIT... sometimes.

modules/quat.lua

@@ -139,6 +139,13 @@ function quaternion:to_axis_angle()
 	local s     = math.sqrt(1-self.w*self.w)
 	local x, y, z
 
+	-- HACK: Why the fuck is this ever NaN?
+	-- I suspect a LuaJIT optimization bug - it doesn't happen in the repl.
+	if angle ~= angle then
+		angle = math.pi * 2
+		s     = 0
+	end
+
 	if s < constants.FLT_EPSILON then
 		x = self.x
 		y = self.y