This change adds a Lua function called gui.setlayermask, which allows Lua
scripts to procedurally modify which render layers are visible. It takes
two arguments: one integer bitfield representing the visibility states for
the main GPU, and the other representing the sub GPU. For example: 0b11111
shows all layers, 0b00000 hides all layers, 0b00001 shows only layer 0
(BG0), 0b10000 shows only layer 5 (OBJ), etc.
Since display layer state is coupled to the frontend, and since frontends
are entirely separate between platforms (i.e., on Windows, toggling
display layer state requires updating the GUI state asl well), this
function is only supported and tested on the Windows build. On MacOS and
Linux, gui.setlayermask will simply return 0 without doing anything.
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