workaround for disable_depth_test

[KrabCode]

draw the GUI as a quad on the main canvas with z set to -1 in a custom vert shader shouldn't need to mess with the hints then and thus leave the app as it was for the next frame, which is way better than the current way of disabling depth test after every draw(), drawing the gui and then enabling the depth test... which may have been disabled by the user intentionally in setup() and not expected to re-enable by itself