2) Change from legacy Cu stack to counting down from top=(F_Cu or 0).
The old Cu stack required knowing the count of Cu layers to make
sense of the layer number when converting to many exported file types.
The new Cu stack is more commonly used, although ours still gives
B_Cu a fixed number.
3) Introduce class LSET and enum LAYER_ID.
4) Change *.kicad_pcb file format version to 4 from 3.
5) Change fixed names Inner1_Cu-Inner14_Cu to In1_Cu-In30_Cu and their
meanings are typically flipped.
6) Moved the #define LAYER_N_* stuff into legacy_plugin.cpp where they
can die a quiet death, and switch to enum LAYER_ID symbols throughout.
7) Removed the LEGACY_PLUGIN::Save() and FootprintSave() functions.
You will need to convert to the format immediately, *.kicad_pcb and
*.kicad_mod (=pretty) since legacy format was never going to know
about 32 Cu layers and additional technical layers and the reversed Cu
stack.
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