Using 8-bit integer math here lead to precision loss for depth copies,
which broke various effects in games, e.g. lens flare in MK:DD.

It's unlikely the console implements this as a floating-point multiply
(fixed-point perhaps), but since we have the float round trip in our
EFB2RAM shaders anyway, it's not going to make things any worse. If we
do rewrite our shaders to use integer math completely, then it might be
worth switching this conversion back to integers.

However, the range of the values (format) should be known, or we should
expand all values out to 24-bits first.

This could cause darker-than-expected EFB copies if clamping was not
enabled, and the user forced EFB copies to RAM only.

Also makes y_scale a dynamic parameter for EFB copies, as it doesn't
make sense to keep it as part of the uid, otherwise we're generating
redundant shaders.

Macros should be all upper-cased. This is also kind of a wart that's
been sticking out for quite a while now (we avoid prefixing
underscores).

It's not often we switch out to draw to the EFB anyway.