new way for hash generation

[jcyuan]

this is what I found recently, really good one: http://byteblacksmith.com/improvements-to-the-canonical-one-liner-glsl-rand-for-opengl-es-2-0/

so I created this PR to solve the issue I posted before: #170, or perhaps #139 is related too? actually everywhere if you want to use random, please use this hash generator instead of the old one.

changes:

1. use new way to generate hash, old one does not work on iOS devices
2. remove the division for pre-multiplied alpha, it's even not enabled before in the for loop, why I need to divide the color by it's alpha at the end? this will cause the pixel too bright.

maybe some other shaders of this project have the old hash method too (it's really common on internet, everywhere....), so you might replace all of them with this new one.

test for changes:
https://jc-space.com/testfilter/test.html?ver=1.1

[jcyuan]

by the way, this piece was already merged into THREE.js official common lib of the shader sys.

test on shadertoy:
https://www.shadertoy.com/view/Wsf3RS

[bigtimebuddy]

Thanks @jcyuan for fixing this