Develop a mask layer's film on its own settings

A layer offered the film's sliders and they moved nothing: its copy of
the node was never given the stock, so it stayed inactive. Film now
works in a layer the way the other adjustments do, as offsets to the
photograph's settings, but blended as settings rather than as results,
since a film is a rendering and cross-fading two developments is not
what a region on a pushed film looks like.

- dr-film bakes no slider. Exposure is a gain in the shader; push
  interpolates the stock's measured processes, one curve row each; the
  print is split at the paper's log exposure, so print exposure is an
  addition between two lookups and exact at any setting. The enlarger
  stays balanced at the photograph's exposure.
- film_sim reads all four settings as uniforms, format one-hot over a
  grain count per format, so every uniform is linear in what it does.
- Operation::blends_settings lets the composer average each overlapping
  layer's uniforms with the global ones by mask weight, the global
  setting taking whatever weight the layers leave, and run the fragment
  once. Three layers at full weight give the mean of their settings.
- The stock picker is hidden on a layer. Only the photograph's exposure
  re-solves the print balance; push, print exposure and format need no
  rebake at all now.

core/dr-pipeline/src/lib.rs

@@ -132,7 +132,9 @@ mod tests {
                 lut: vec![[0.5, 0.5, 0.5]; 32 * 32 * 32],
                 density_max: 3.0,
                 lut_size: 32,
-                grain_particles: [0.0; 3],
+                grain_particles: [[0.0; 3]; crate::ops::film_sim::FORMAT_COUNT],
+                push_stations: vec![0.0],
+                paper: None,
                 grain_density_max: [3.0; 3],
                 grain_uniformity: 0.97,
             },