Minetest_game: Add heart_gone.png and bubble_gone.png

[Wuzzy2]

I suggest to add heart_gone.png and bubble_gone.png (see lua_api.txt).

Make these images grayed-out versions of heart.png and bubble.png.

This simple change makes the statbars so much more user-friendly.

[An0n3m0us]

Note that this should be done after #2655 is worked out, because heart.png and bubble.png might need to change.

I decided to try it out; with a bit of grayscale conversion and 'Color to Alpha' to give it some transparency:

It needs to also work well at night; which my ones don't do at 50% screen brightness, if that means anything :P

[An0n3m0us]

I went with less alpha and got this result:
Day:

Night:

I'm not too keen on how the heart gone icon look like at night; but at least they are visible.

[sfan5]

Why have transparency at all?

[An0n3m0us]

Well, a solid color would look a little odd, wouldn't it? It's not too bad; but for some reason it just looks like a texturepack rather than an empty health bar.

Actually, it probably needs a different icon rather than a grayscale of the current heart icon.
Maybe like this:

[paramat]

Attend to this after #2655 EDIT: That issue is now resolved.
Solid textures seem ok to me as long as they are suitably designed (dark , greyscale), it will be obvious they are 'gone' due to the presence of the 'not gone' textures.

[An0n3m0us]

I'm just creating a few examples for now.
Yeah they need to be obvious, unlike the first grayscale ones I showed.

[Wuzzy2]

Feel free to “steal” from [hudbars].

[paramat]

'Gone' icons should be derived from the new textures added in #2669
'Gone hearts' should probably be opaque but 'gone bubbles' should probably have transparency.