Hacking up the BMP brush to remove conflicting blend tiles in the map.

When painting with the BMP brush we expect the same result as if BMPToTMX
was run in (and around) the painted area.  However, blend tiles in the map
were not being removed.
BmpBlender doesn't put a tile in the map if the current map tile is wanted.

The map had a blend tile that should have been there.  The blender then
put another tile in the same layer that the map tile was supposed to be
overriding according to the normal blending rules.

A side effect of this change is that the blender now blends 2 squares adjacent
to where pixels are, rather than 1.
$$PWD *is* the directory of the .pri file, not the .pro file including it.

At least the docs say so...