Backport changes and extensions from Minetest

[numberZero]

These were added before forking Irrlicht. Currently there is no point keeping them separate.

See also: minetest/minetest#13227

[sfan5]

This should be all that's left:

• https://github.com/minetest/minetest/tree/master/src/irrlicht_changes
• https://github.com/minetest/minetest/tree/master/src/gui

Not sure if it makes sense to move the src/gui/ stuff to Irrlicht either since it might be too integrated to MT.
In fact it might not make sense to move anything back since Irrlicht is supposed to merged into MT not the other way around.

[numberZero]

If the GUI stuff is all that is there, it may indeed make more sense to to move in the opposite direction: move all GUI stuff that MT still uses into MT, and drop GUI part here altogether.

[numberZero]

There is also src/irrlichttypes.h:

• irr::core::roundingError<s16> ()
• *_MIN, *_MAX

src/irrlicht_changes is GUI stuff only so it should likely merged into src/gui.

[Desour]

irr_ptr would be good to have in irrlicht.

[numberZero]

irr_ptr would be good to have in irrlicht.

Makes sense. For the API I’d wait for The Merge but for internal use, it’s OK to start using it now IMO. My grep suggests there are 287 drop calls and 89 grab calls awaiting replacement.