Fix missing item images clipping in formspecs

[pyrollo]

When using item_image_button in formspec with a node, item image is not clipped at all.

This PR makes drawItemStack clip drawn image against clip rectangle even with 3D rendered items.

How to test

You can easily test result of this PR with this Lua code :

minetest.register_chatcommand("t", {
	params = "",
	description = "Test",
	func = function(name, param)
		minetest.show_formspec(name, "test", [[size[5,5]
		item_image_button[0,-0.75;1,1;default:stone;test;]
		item_image_button[1,-0.5;1,1;default:stone;test;]
		item_image_button[2,-0.25;1,1;default:stone;test;]
		item_image_button[3,0;1,1;default:stone;test;]
		item_image_button[1,5;1,1;default:stone;test;]
		item_image_button[-0.75,1;1,1;default:stone;test;]
		item_image_button[-0.5,2;1,1;default:stone;test;]
		item_image_button[-0.25,3;1,1;default:stone;test;]
		item_image_button[4.5,1;1,1;default:stone;test;]
		item_image_button[4.75,2;1,1;default:stone;test;]
		item_image_button[5,3;1,1;default:stone;test;]
		]])
	end,
})

Result before the modification:

Result after it: