For fixing a previous issue state.canvas_texscreen_used was reset to false at the start of each render_joined_item. This was causing a later shader that used SCREEN_TEXTURE to force recapturing the back buffer immediately prior to use, which we don't want.
This PR preserves the state across joined items, and also prevents joining of items that copy the back buffer as this may be problematic.
It turns out that the original issue that needed the line is now fixed, and the later issue is also fixed by removing it.
7c4cf1c043