MilkDrop fidelity: stage rule, fixed composite and defaults

[CaYatur]

What

Follow-ups from the MilkDrop fidelity audit, checked against Nullsoft's code (jecassis/foo_vis_milk2 5b44cea) and, for the fixed pipeline's blend passes, the D3D9 code it was ported from (BeatDrop 53d83ee). Everything is behind the fidelity switch; with it off the legacy paths are unchanged.

• Stages by version. MilkDrop picks the warp and composite shaders from the file's version, not from whether shader text exists.
  • No MILKDROP_PRESET_VERSION, or one below 200, means a MilkDrop 1 preset, and its shader text is ignored.
  • Version 200 uses PSVERSION; 201 and above use PSVERSION_WARP and PSVERSION_COMP, each 2 when missing.
  • A stage with a version but no text gets the shader MilkDrop writes at load time, with the file's values baked in.
  • Switching fidelity rebuilds the running preset's stages.
• Fixed composite (2,128 corpus presets). Brighten is 1−(1−c)² and solarize is 2c(1−c); the engine used sqrt(c) and 4c(1−c), the formulas of the generated shader. Echo orientation is (int)x % 4. Gamma below 1 is not applied while echo is on. When two presets whose echoes point different ways blend, the echo fades out and back in instead of flipping.
• Defaults for keys a file does not write: decay 0.98, gamma 2.0, echo zoom 2.0, wave alpha 0.8, mv_l 0.9, grid 12×9, border sizes 0.01, inner border colour 0.25. Init code sees them too, and a centre of 0 stays in the corner.
• Our own presets write fWaveAlpha, the key MilkDrop reads. The builtins and the generator wrote wave_a, so MilkDrop drew their wave at its default 0.8. Nothing changes on screen in our engine; the generator's golden hashes were renewed and its version kept.
• Mash-ups follow the stage rule. The mash-up rule version goes to 2, so saving a recipe from before this change again writes a new file instead of overwriting the old one.

Measured

• Rendered cases: 66 of 66 flag, gamma and orientation cases are within 2/255 of MilkDrop's formula, with fidelity on and off.
• 900-preset sample with the texture pack: the same classes before and after (882 clean, 12 blown, 4 frozen, 2 black).
• Last 10 of 60 frames against main:
  • 99 of 120 brighten presets changed (median 3.8%).
  • 63 of 83 solarize presets changed; median brightness went from 0.291 to 0.112.
  • 23 of 49 presets with a centre of 0 changed, and 7 of 38 that write echo_orient per frame.
• Mash-up part tests: they agree with the engine's stage choice for every corpus preset and part.

Found here, fixed in the next PR

• Hue colour in preset shaders. MilkDrop gives every composite shader the full hue colour ("since we don't know if shader uses it or not"). fShader scales it only on the fixed path and in the generated shader. The hue_shader change in 52c7391 (MilkDrop Phase 2: visual rendering quality, user textures (img.ini), motion vectors, and smooth preset transitions #560) applied fShader to preset shaders too. With fidelity on, 914 corpus presets lost the colour and 36 got part of it.

• Keys that fall back to other values when missing:

  • wave_r, wave_g, wave_b, wave_x and wave_y: MilkDrop reads them as rot's value, which is 0.
  • Wave smoothing is 0.75, and the volume fade runs from 0.75 to 0.95.
  • Shape colours.

  No corpus preset and none of ours leaves them out.

Tests

• 14 new tests and 8 updated test files; 36 of 36 mutations are caught.
• npm test: 2181 of 2181 pass, and each commit passes on its own. The changed files also pass under Linux emulation.
• npm run smoke: PASS, with the userData checksums unchanged.

Refs #580
Refs #579
Refs #560