od-unix: ship starter filter presets

od-unix: ship starter filter presets

The Filter page's preset list was empty until the user saved their own
(GitHub issue #4). Add a set of bundled presets built from the portable
filter controls the Unix GLSL pipeline implements (Sharp Pixels, Soft
Bilinear, CRT Scanlines, Warm TV), installed under the shared data
directory on Linux and into the app bundle resources on macOS.

The preset combo lists user presets and bundled presets together; the
user directory wins on name collisions, saving always writes to the
user directory, and loading falls back to the bundled file when no user
preset of that name exists.