added Engine::Quad::setColor() and ag::quad::setColor()

tests/managed_objects.lua

@@ -58,8 +58,10 @@ function key_down_event(key)
     elseif (key == "minus") then
         ag.setCursorVisible(not ag.getCursorVisible())
     elseif (key == "d") then
-        q = ag.createQuad(3, 3, 0, 1, 0, 0, 0, 1, 0)
+        q = ag.createQuad(0.5, 3, 0, 1, 0, 0, 0, 1, 0)
+        q:setColor(0, 0, 1, 0.6)
         q:setOffset(1)
+        q:setBlending(true)
     end
 end