Allow Shadertoy shaders to be used

[llewelld]

Test out whether shaders uploaded to shadertoy.com can be used. If not, figure out a way that they can be transferred over easily.

https://www.shadertoy.com

[llewelld]

It turns out, for many of the shaders, the transfer if pretty straightforward.

Here's an example of a rather excellent shader by Inigo Quilez:

http://www.flypig.co.uk/shaders/test.html?shader=shadertoy-iq.txt

using the original code from Shadertoy.com: https://www.shadertoy.com/view/Xds3zN

The change just involved adding some conversion variables at the top:

varying vec2 vTextureCoord;
uniform highp float time;
uniform float width;
uniform float height;
vec3 iResolution;
float iGlobalTime;
vec2 iMouse;

and a wrapper function to perform the conversions at the bottom:

void main () {
    vec4 fragColor;
    vec2 fragCoord = vTextureCoord * vec2(width, height);
    iResolution = vec3 (width, height, width / height);
    iGlobalTime = time / 1000.0;
    iMouse = vec2(0.0, 0.0);

    mainImage(fragColor, fragCoord);

    gl_FragColor = fragColor;
    gl_FragColor.a = 1.0;
}

This could be integrated directly into shaderback.js with a helper function.

[llewelld]

Shadertoy has an API that could be used: https://www.shadertoy.com/api

[llewelld]

As of version 0.03, commit 5c21c80, the Time, vTextureCoord, width and height inputs have now been refactored to iGlobalTime, fragCoord and iResolution. These now match the Shadertoy inputs.