MP4Splitter: fixed playback for same 3gp with AMR sound (fixed 4496) (ticket #3079)
Audio Switcher: Improve normalization algorithm.

Use a much better regain strategy (similar to ffdshow's) and fix the regain rate to be linear rather than completely exponential.

This commit fixes #1936.
Audio switcher: Use percentage instead of dB for the boost.

Internally a percentage was already used and it is much easier for the user to understand a boost expressed in percentage.
Audio Switcher: Add a setting to change the max normalization factor.

Use a max normalization factor of 4 by default instead of 10 to avoid huge volume variations.

Refs: #1936.
Bump version to 1.6.8.
run astyle
update lcms to 5bf9e43
cosmetics
VSFilter: move the AboutDlg version to a new define and use that
AboutDlg.cpp: use the MPC_VERSION_ARCH define
Reduce variable scope.
remove unneeded variable