Area::write_texture_2d is broken for multiple draw calls

[janpaul123]

If you do something like this multiple times in a row:

let area = cx.add_instances(&MY_SHADER, …)
area.write_texture_2d(…)

then the draw calls will get batched, and it will only get the last texture (the others will get overwritten).

The workaround would be to make a dummy cx.add_instances call in between.

Found by @pankdm, who also showcased the workaround here: #159