GPU drop-shadow: the CSS filter set is complete on the device

The drop-shadow filter joins the GPU path, completing the CSS filter set. The content's silhouette is tinted and offset, its blur - the expensive part - runs on the GPU, and the shadow composites underneath the content exactly as the software renderer does, matching within a rounding level. Every CSS filter op - grayscale, brightness, contrast, invert, saturate, sepia, hue-rotate, opacity, blur, and drop-shadow - now runs its per-pixel work on the device, alongside the full 2D paint set, clips, and transforms. The GPU aspect moves 42 to 44. What remains toward full acceleration is making the GPU renderer the default live path, WebGL, and video decode. The overall holds at 57.