Butterfly animation.
Tested on Oculus Go (with touchpad) and VR BOX2 (with joystick).
For tests on a desktop (Windows 10 + Chrome) the "inline" webxr session and
A,S,D,W,↑,↓ keys for navigation are used.
Set "anisotropy" before "Enter WebXR". EXT_texture_filter_anisotropic extension and
gl.LINEAR_MIPMAP_LINEAR texture filtering mode are used.
Butterfly Function
(x2 - y2) sin(x + y)/(x2 + y2) is used
for prosedural textures. It is "rotated" by x = (u + v)/2, y = (u - v)/2 transformation as
2uv sin(u)/(u2 + v2).
WebXR samplesupdated 11 Apr 2020