GPU image blit: pictures draw on the device, pixel-for-pixel

Images now rasterise on the GPU. A picture uploads to the device as a texture and is drawn with the exact same nearest-neighbour mapping the software rasteriser uses, so an opaque image scaled to its box comes out bit-for-bit identical to the software path - not close, identical. Rounded-corner images (avatars, thumbnails) reuse the same one-pixel edge ramp and match within a level. With rects, rounded rects, borders, gradients, and now images on the GPU, everything a page paints except text and WebGL is hardware-rasterised. The GPU aspect moves 14 to 17; a glyph atlas for text is the next and largest step. The overall holds at 57.