in can_dig (e.g. if it's a technic quarry or something)

(drop the glass on the ground, if necessary)

(e.g. front of sofa faces you)

to use on_rightclick instead, for consistency with other parts of minetest,
minetest_game, etc.

requires unified dyes commit b0ccff16 or later

side effect:

I added a third shade of the formerly-white checker background
that alternates with the colored overlay.  The colored overlay
follows the standard full Unified Dyes palette.

* Don't trigger global variable warning.
* Change form name to avoid possible bugs in the future.
* Actually (opt-)depend on screwdriver mod.