Wrote a quick fix for MC-13206.
In vanilla minecraft whenever an animation updates, the entire texturesheet
has to be updated. For HD texturepacks or when there's a lot of mods, the
terrain texturesheet is huge, so the performance loss is huge. This update
changes Texture.CopyFrom to directly modify certain texturesheets using
glTexSubImage2D, instead of triggering a full update with glTexImage2D.

Signed-off-by: retep998 <retep998@verizon.net>