Audio Switcher: Improve normalization process.

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

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.
Add the ability to close a media instead of pausing/stopping at the EOF.
Make "Sharpen Complex 2" shader compatible with Pixel Shader 2.0.
Output normalization factor just for tests.