opus

Cast a factor to maintain precision on 16 bit systems.

The multiply would overflow with 16 bit ints. Thanks
to Riccardo Micci for pointing out the issue. Thanks
to Tim Terriberry for the valid range of the decay
argument.

Note that ft is unsigned, but always less than 32736,
so we could use a 16 bit signed type here if it allows
the compiler to produce faster code (with signed 16*16
and 16*32 multiplies). In the absense of actual cycle
counts from a real platform, I've left it as an unsigned
for the sake of readability.

For similar reasons we cast (16384-decay) to a signed
integer even though it is also always positive.