MSVS profiler shows that the encoder spends too much time inside format_input()
when the input is 24-bit. Increases encoding speed:

FLAC -5: from 27.1 to 24.2 seconds
FLAC -8: from 76.2 to 73.1 seconds
(MSVS 2010, 32-bit flac.exe, 24-bit stereo input file)

For GCC compiles the encoding speed remains the same. I suspect that GCC is
smart enough to use strict aliasing rule to optimize the code, and MSVS doesn't
even know about it.

Path-from: lvqcl <lvqcl.mail@gmail.com>