Mask ids come unchecked from develop.merge, sidecars (lc:settings) and
pastes. With a mask id of u32::MAX, next_mask_id() and Preset::apply
added 1 to it: debug builds panicked, release builds wrapped to 0, so
the second new mask got the same id as the first.
next_mask_id() now falls back to the lowest id no mask uses when the
highest is at the limit, and Preset::apply takes each preset mask's id
from it instead of counting on its own, so mask.add, mask.brushStroke,
mask.duplicate and presets with masks all get unused ids.
Bruno-Alumn
bc12ec8e40