Support Shadertoy channel inputs

[kfish]

Shadertoy defines the following inputs for data channels (music, textures, videos):

uniform float     iChannelTime[4];       // channel playback time (in seconds)
uniform vec3      iChannelResolution[4]; // channel resolution (in pixels)
uniform samplerXX iChannel0..3;          // input channel. XX=2D/Cube

Probably the first step would be to support sampler2D texture inputs, which are the most commonly used input channel, before dealing with music (which would require Web Audio support in Elm) and video.