shaders/dithering: work around d3d11 shader compiler bug

The d3d11 compiler really doesn't like this for some reason, so rewrite it using a conditional instead of a continue statement. This is obviously a compiler bug because the two formulations are logically 100% equivalent. But whatever, it's not like we get to rewrite Microsoft's infrastructure.

Fixes: #228 (closed) Fixes: https://github.com/mpv-player/mpv/issues/10676