Add pack/unpack for data masks

[GriffinRichards]

• Adds packing/unpacking macros for map data and metatile attribute data. There were 2 intentions here:
  • At the moment it's difficult to reassign bits on the right side of MAPGRID_METATILE_ID_MASK / METATILE_ATTR_BEHAVIOR_MASK because they have no shift value to update, and adding a shift value makes some simple code uglier. This adds a shift value for them and hides the extra complication behind the macros.
  • This should hopefully make it easier for someone downstream to utilize something like __builtin_clz to get rid of the explicitly-defined shifts altogether.
• Adds MAPGRID_IMPASSABLE as an alias for MAPGRID_COLLISION_MASK when it's used to set impassability for a map space.
• Adds a BUGFIX in decoration.c related to layer type usage (arguably not a bug, but a pitfall we have that would only be fixed by making the documentation worse)

[mrgriffin]

Great change! Just one comment, GetLayerType implicitly adds NUM_TILES_IN_PRIMARY because the decoration tiles are always in reference to the secondary tileset's tiles... But I think that fact might be too subtle in this code:

layerType = GetLayerType(decoration->tiles[(mapY - 1 - i) * mapX + j]);

My weakly-held opinion is that maybe the NUM_TILES_IN_PRIMARY + part could come back, or perhaps renaming decoration->tiles to something like decoration->secondaryTiles?