Improve soft mask composition performance (bug 2033095)

Prepare reusable soft-mask canvases for filtered and backdrop-dependent masks,
and use a faster destination-in composition path where possible.
Handle Alpha SMask /BC correctly, preserve OOB alpha behavior, and mirror canvas path
operations needed while rendering inside soft-mask mode (mirrored clip was buggy).

Add reftest PDFs covering Alpha masks, transfer functions, backdrop/OOB
alpha, and the optimized composition paths.

Author: calixteman