TextureState

I hope this pseudo code sample will help make it clearer how activeTexture, bindTexture, and the various texture functions in WebGL effect the state of textures.

Imagine textures are defined like this

   Texture2D = function() {
     this.levels = [];
     this.state = {};
   };

And the WebGLRenderingContext is defined like this

   WebGLRenderingContext = function() {
     // activeTexture is just an index into textureUnits
     this.activeTexture = 0;
 
     // these are the textureUnits for this context.
     this.textureUnits = [
       { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
       { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
       { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
       ...
     ];
   };

Then activeTexture would be implemented like this

   WebGLRenderingContext.prototype.activeTexture = function(texture) {
     this.activeTexture = texture - this.TEXTURE0;
   };

bindTexture would be implemented like this

   WebGLRenderingContext.prototype.bindTexture = function(target, texture) {
     this.textureUnits[this.activeTexture][target] = texture;
   };

And texture functions like texParameter or texImage2D would be implemented like this

   WebGLRenderingContext.prototype.texParameteri = function(target, pname, value) {
     var texture = this.textureUnits[this.activeTexture][target];
     texture.state[pname] = value;
   };