Waldorf Quantum/Iridium: write up to four layers, the layout of the MK2 generation

[douglas-carmichael]

Stacked on #426 and #425 and rebased on main (it needs the entries which name the samples of the later layers in the first layer); the diff shows all three until those land.

The 'Use a second layer for more than 3 groups' check box becomes a Maximum number of layers drop-down with 1, 2, 3 or 4 layers (-p QPATLayers=1|2|3|4 on the command line; a stored setting and the old parameter of the check box are taken over). The drop-down is built like the other ones of the settings panes, with its label. With 3 or 4, a source with more than 6 groups is written with up to 9 or 12 multi-samples in the layout the Iridium MK2 stores its own patches in: the layer count 2 at offset 428, the format version 15 and always four stored layers, of which the unused ones are switched off - LayerActive 0, no oscillators, no sample maps - exactly like the 304 version-15 files of the corpus, which store four layers for every dual-layer patch. The loader of the device clears the file offsets of the layers 3 and 4 in files up to the version 14 and checks the offsets of all four layers when it loads a count-2 file, which is why the layout needs the version 15 and the two extra blobs.

Only as many layers as the groups need are used, so a source with up to 6 groups is still written as the 2-layer patch of the version 14 which every instrument of the family plays; a 4-group source gives a byte-identical file to the one the check box produced. The samples of all later layers are named in the first layer for the importer of the device (#426).

Checked:

• Stacks of 7 and 12 groups written with 4 layers: four contiguous layers, the offsets in every header equal the layer starts, layer 4 of the 7-group stack is switched off, every later-layer sample is named in the first layer; the reader reads them back as 3 and 4 multi-samples.
• The QPAT test corpus: 19 of 19.
• Hardware: on an Iridium MK2 with OS 4.0.6, a written four-layer patch with one sine pitch per layer plays all four pitches, and a three-layer patch with its switched-off fourth layer plays its three. The first generation of the instruments holds two layers - its manuals switch 'between the two layers by tapping on the Layer 1 or Layer 2 button', and Waldorf describes the MK2 upgrade as raising the count from 2 to 4 - so the option says 'MK2 generation only'.