Check file headers instead of filename regex to determine image format

[ebogo1]

This came up in #9513 - files like Material.js have code along the lines of:

var ktx2Regex = /\.ktx2$/i;

function createTexture2DUpdateFunction(uniformId) {
  ...

  if (ktx2Regex.test(resource.url)) {
    promise = loadKTX2(resource.url);
  } else {
    promise = resource.fetchImage();
  }

  ...
}

We should update these checks to check .ktx2 header info since not all files will have the .ktx2 extension.