For Overlay::AllMasks, the masks buffer already holds each visible mask's
alpha back-to-back in the same order the single-mask overlay uses
(cx.slice(all, mi*n, n)). Read the whole buffer once with cx.read instead
of one synchronous read_plane per mask, then split it into per-mask Planes
on the CPU. Same bytes, but the pipeline stalls once instead of N times.
树洋 陈
9fc2e69328