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; For unknown reasons, the transparency of the portal was made uniform rather than based on the blue channel in the 1.14 texture update (a change not unique to the portal, as both of the water animations have this error as well).