By default Aseprite will not try to match flipped versions of the
tiles (as it requires more CPU), but when we create a tileset we can
specify which flips can be matched automatically (new
Tileset::matchFlags() property).
These flags are just for the Auto mode, if we manually insert a
flipped tile, that is always supported, even when the matchFlags() are
not specified.
In this way we always have an empty tile available in the drawing
process. We've also added the Tileset::firstVisibleIndex field to
change the visible index of the tile 1 so we can offset the visible
number by the user (just as a visual aid / simulate old tilesets with
index=0=non-empty tile).
