Procedural animated texture generation/Nether portals

Due to the large number of Atan2 and Sine operations, all frames of animation (a total of 32, at a resolution of 16×16) for the portal block are generated once at-startup and stored into an internal animation strip; The random shimmer is the same every time, as the game always uses a random number generator with a seed of 100L.

To generate one frame of the nether portal animation:

The portal block is the only one of the procedural textures that is ported 1:1 in the 1.5 resource pack changes; Unlike the water animations, the alpha channel was not made uniform in the 1.15 texture update.