Add Focus pull, a video effect that pulls Blur in and out of focus

Blur's box blur moves to effects/box_blur.hpp so both share it; Blur renders
exactly as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -41,6 +41,11 @@ commit that made it. The interface shows this file under Changelog in its menu.
   through its image, or sliding sideways, while the strips stay put. On the
   command line it is `-e striproll` with `--roll-speed`, `--roll-axis`,
   `--roll-direction` and the rest.
+- Focus pull, a video effect: a lens being focused, either the whole
+  picture blurring as Blur blurs it and coming back sharp, or a band of
+  sharpness moving down the picture and back up. On the command line it is
+  `-e focuspull` with `--focus-mode`, `--focus-radius`, `--focus-band` and
+  `--focus-speed`.
 - A Quality choice under Video on Setup: Small file, Standard or High, for
   how the saved video is encoded. It is kept in the project, and on the
   command line it is `--video-quality small|standard|high`. Standard writes