MilkDrop: drive the lights from the colours MilkDrop draws

[caganturgut]

Lighting follows a selected colour theme, which works, but there is no way to drive the lights from what MilkDrop is actually drawing.

An option in the MilkDrop settings: the lights take their colours from the current MilkDrop picture in real time, changing continuously with the preset.

[CaYatur]

Done in a16439d (#591).

New color source, MilkDrop picture (live), in Lighting › Color Source and as a shortcut in the MilkDrop panel (Light Colors), which remembers the previous source and restores it. The current frame is read about 30 times a second: shrunk to 64×16 and cut into eight slices from left to right, each slice's color weighted by brightness so a small bright detail on a dark background still counts, hue kept, brightness left to the lighting mode. It fills the same sampled palette the background used, so the background-based modes follow MilkDrop too; with no MilkDrop in the scene it falls back to the background.

Dynamic Lighting, OpenRGB and Art-Net all take it. OpenRGB also uses the sampled palette the way Dynamic Lighting does now; before, it drew the configured gradient.

Measured over Art-Net sent to localhost, with Dynamic Lighting and OpenRGB off so no real lights were driven, and a preset red on the left and blue on the right: the first four of eight fixtures came out red and the last four blue. Before, all eight showed the fallback color and the panel received no palette. The window ran at 75.2 Hz with the sampling and 75.0 Hz without.

[CaYatur]

A follow-up is on main via #608 (55b0a41). The colour sampler read its 64x16 canvas with getImageData about 30 times a second, and Chromium warned that the canvas should be made for frequent reading. The smoke test caught that warning once lights were set to take MilkDrop's colours.

Making the canvas CPU-backed would have pulled the whole frame back every time, about 8 MB at 1080p. Instead, the downscale stays on the GPU canvas and only the 4 KB result is copied into a second canvas made for reading. The colours are computed exactly as before; the existing tests pass unchanged, and one more checks where the reading happens.