Audio Switcher: Fix custom channel mapping after 14bc5b28faaf7fe59c9a52d73c237ea47402e8c6.

The output was silent when custom channel mapping was enabled.

This commit fixes #3257.
Internal LAVFilters: Add some classes to handle the filters.
Internal LAVFilters: Modify the graph manager to use LAVFilters.

Note that currently the "internal" LAVFilters share their settings with their standalone version.
Internal LAVFilters: Basic modifications to make the UI consistent.
Internal LAVFilters: Use the default settings instead of sharing the settings with the standalone version of the filters.
Internal filters: Add new menu entries to enable/disable all audio/video decoders.
DSUtil: Add some functions to get the path of a registered filter from its CLSID.
Internal LAVFilters: Give more flexibility for the path from where to load the filters.

If a "LAVFilters" folder exists together with the executable, it will always be preferred. However if the filters aren't available in that folder but are registered on the computer, they will be loaded from the registered path if that option was enabled on compile time.

This option is disabled by default.
Internal LAVFilters: Check the version of LAVFilters before using it.
Internal LAVFilters: Remove now irrelevant FFmpeg version from the About dialog.
Internal LAVFilters: Stop building old internal filters for MPC-HC.
Internal LAVFilters: Add LAVFilters as a submodule and build them when compiling MPC-HC.
Internal LAVFilters: Include LAVFilters in the packages.
Internal LAVFilters: Speed-up compilation by skipping ffmpeg configure whenever possible.
Internal LAVFilters: Use our own internal settings for LAVFilters.

Note that currently our internal default settings are the same as LAVFilters' and that there is no user interface yet to control the settings.
Internal LAVFilters: Keep track of LAVFilters instances in runtime mode.
Internal LAVFilters: Save LAVFilters settings after closing the filter property window opened via Play > Filters menu.
Internal LAVFilters: Hide the "Formats" property page for the internal instances.
Internal LAVFilters: Let the user access the filters property pages from the "Internal filters" option page.
Graph builder: Prefer a filter loaded directly from an external file only if it has the same merit.

The merit can be tweaked in the "External filters" page so there is no reason to always prefer the filters loaded this way without considering their merit.