Textures API for adding block and item textures in versions 1.4.7 and below

[SpaceWalkerRS]

1.5 overhauled the block and item texture system. Each block/item sprite became its own file, which the game would stitch together into an atlas at runtime. The goal of this API is allow for something similar in versions 1.4.7 and below.

This issue serves as a place to discuss different approaches before a path is chosen and work on the API is started.

[SpaceWalkerRS]

Like the rest of OSL, this API should be minimally invasive. That means texture packs that modify terrain.png ought to work if OSL is installed.

One approach would be implement expandable atlases like in later versions, and replace the vanilla atlases with the expandable ones. For blocks, the terrain.png texture would be stitched into the atlas first, allowing vanilla blocks' textures to work without any further modifications, as their uv coordinates would be the same still. Custom blocks would have to be registered similar to how they are in 1.5-1.7.