new mod: common.layers

[o7Moon]

writes:

• u16 layer count

for each layer:

• 3 bytes for the layer's 24 bit color.

for each line (simlines come first, then scenery):

• u16 for the line's layer

layer names are left to a separate mod. if an implementation wants to display names and that other mod isn't present it can load the layer names as "Base Layer" for ID 0, followed by "Layer N" for all other IDs where N is the ID.

layer editability and visibility are also left to separate mods, and assumed both true if those mods aren't present.