Basic ShaderToy shader support

[osnr]

IMG_5703.MOV

Basic example (same as https://www.shadertoy.com/new):

Wish $this draws toy shader {
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = fragCoord/iResolution.xy;

    // Time varying pixel color
    vec3 col = 0.5 + 0.5*cos(iTime+uv.xyx+vec3(0,2,4));

    // Output to screen
    fragColor = vec4(col,1.0);
}
}

ShaderToy code should run verbatim as long as it only uses iResolution and iTime (no channels, iFrame, iMouse, or iFrameRate support yet).

Tested and working:

• https://www.shadertoy.com/view/msjGzw
• https://www.shadertoy.com/view/4tc3DX
• https://www.shadertoy.com/view/XlSBz3

This is not very flexible right now -- you can't add additional uniforms -- but I like the idea of maintaining verbatim compatibility so we can use the existing library of ShaderToy. Can try to make it extensible later while maintaining compatibility (maybe use reflection to find declared uniforms)

Fixes #106 / May supersede #270