Complete rewrite of the Scale2x (AdvanceMAME2x) filter. The old version was
based on an implementation licensed under the GPL, which is incompatible with
our license. Like the version it replaces, this rewrite has a fast MMX path
for x86 machines and a C path for everything else.
There's still a ton of GPL-encumbered code in the other scaling filters, but
this is at least a start.
