GPU shadows and conic gradients: the fallback set shrinks

Two more paint primitives move onto the GPU, so the unified renderer hands fewer things back to the software path. Outset box shadows - the soft drop shadow under every card and button - now rasterise on the device: a shader reproduces the software renderer's exact rounded-rect distance field, blur falloff, and the way it carves out the element's own box, matching within a level or two. Conic gradients join linear and radial on the GPU too. With shadows and all three gradient kinds now on the device, a real card - background, border, gradient, shadow, and text - draws almost entirely on the GPU, with only transforms, clips, and filters still on software. The GPU aspect moves 27 to 31. The overall holds at 57.