I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

composeFn = composeSMaskLuminosity;
    } else {
      composeFn = composeSMaskAlpha;
    }
    
    const PIXELS_TO_PROCESS = 1048576;
    const chunkSize = Math.min(height, Math.ceil(PIXELS_TO_PROCESS / width));
    for (let row = 0; row < height; row += chunkSize) {
      const chunkHeight = Math.min(chunkSize, height - row);
      const maskData = maskCtx.getImageData(0, row, width, chunkHeight);
      const layerData = layerCtx.getImageData(0, row, width, chunkHeight);
      if (hasBackdrop) {
        composeSMaskBackdrop(maskData.data, r0, g0, b0);
      }
      composeFn(maskData.data, layerData.data, transferMap);
      maskCtx.putImageData(layerData, 0, row);
    }
  }
  function composeSMask(ctx, smask, layerCtx, webGLContext) {
    const mask = smask.canvas;
    const maskCtx = smask.context;
    ctx.setTransform(
      smask.scaleX,
      0,
      0,
      smask.scaleY,
      smask.offsetX,
      smask.offsetY
    );
    const backdrop = smask.backdrop || null;