add sprite and rendering to engine js

scripts/particle.js

@@ -108,7 +108,7 @@ var make_emitter = function()
 {
   var e = Object.create(emitter);
   e.ssbo = render.make_textssbo();
-  e.shape = shape.quad;
+  e.shape = shape.centered_quad;
   e.shader = render.make_shader("shaders/baseparticle.cg");
   e.dead = [];
   return e;