B&W fast-render mode for scroll-heavy use

[exp78]

From a Reddit comment (starkruzr): an option to render in black & white only + a faster display mode when a lot of scrolling/paging is needed.

Rationale: the Paper Pro's Gallery 3 (ACeP) panel resolves color slower than monochrome, so a forced-grayscale render plus a faster waveform would speed up page turns further.

Ideas: settings-page toggle; grayscale filter in the WpeView paint path; faster waveform selection in the epaper QPA during continuous paging; keep color for full/idle refreshes.

[exp78]

Implemented in 365402e (master). Settings page gets a "B&W fast mode" toggle: rmweb presents grayscale frames, and since Gallery 3 develops mono content fully on the fast waveform (colour needs the slow multi-pass flash), page turns land clean and quick. Will ship with the next release.