Some backends like GCW0 do no support graphics >320x240 due to
the hardware limitation (downscaling is possible but it will ruin
the pixel hunting which is often part of the gameplay).
Instead of manually updating the list of engines, we now introduce
a new dependency.
I marked all relevant engines, but some, like tinsel, require more
work with putting their relevant high-res games under USE_HIGHRES
define.
This is the first part of allowing engines to be added dynamically.
They are placed into a folder in engines/ which must contain a file
named "configure.engine" to add the engine, which is pulled into the
top level configure script automatically.
