Durability bar cannot be retextured

The bug

It is not possible to change how the durability bar looks via a resource pack - the effect has been generated purely by code ever since durability was implemented in Indev.

This (and other recent reports) may count as feature requests, but I'm not sure where the line between that and genuine issues even is, especially given recent changes which have made previously-hardcoded graphics such as the pause menu background, text fields and scroll bars retexturable.

How to fix

Introduce texture files for the durability bar - one texture for the background alongside textures for each progress iteration (1 to 14 pixels) would suffice. Ideally these should be at least 16x16 so that the bar can be positioned anywhere desired within the slot. There could alternatively only be one progress texture which would be colored by the game, although this limits resource pack freedom.

Fixing this could also fix issues like MC-265206 and MC-265207.