Save/load texture atlas for caching to improve performance

[deathcap]

The complete process of generating the texture atlas is fairly expensive (including extracting the texture .png files from the resource pack .zip archives, which is not cached either: deathcap/artpacks#15), so it ought to be possible to save it and load a precomputed stitched atlas (including all the mipmap levels) for better performance.

There is existing code when the debug option is enabled to log the canvases to the document:

  if (showLevels) {
    // add each mip level to the page for debugging TODO: refactor with rect-mip-map demo
    pyramid.forEach(function(level, i) {
      var img = new Image();
      img.src = savePixels(level, 'canvas').toDataURL();
      img.style.border = '1px dotted black';
      document.body.appendChild(document.createElement('br'));
      document.body.appendChild(img);
      document.body.appendChild(document.createTextNode(' level #'+i+' ('+img.width+'x'+img.height+')'));
    });
  }