fmbtgti: .fmbtoirrc always loaded only from png dir

Given bitmaPath "/foo", tapBitmap("bar/i.png") used .fmbtoirrc from
/foo. This was confusing, especially if .fmbtoirrc had "includedir" in
it. Now .fmbtoirrc is loaded from /foo/bar, and includedir prefixes
"i.png", not "bar/i.png". This results in always the same behaviour
indepently from the way how i.png is referred to (1: absolute path, 2:
relative path, 3: only by "i.png" and with /foo/bar in bitmap path).