ffmpeg can now use the x265 multi-library interface to make a runtime
selection between a number of libx265 libraries (perhaps 8bpp and 16bpp).

ffmpeg will link to one build of libx265 (statically or
dynamically) and this linked version of libx265 will support one
bit-depth (8 or 10 bits). At runtime, ffmpeg now has the option to request the
encoder to use a different bit depth(8 or 10). If the requested bitdepth
is zero, or if it matches the bitdepth of the system default libx265 (the
currently linked library), then this library will be used for encode.
If ffmpeg requests a different bit-depth, the linked libx265 will attempt
to dynamically bind a shared library with the requested bit-depth from the install
location (default or user-specified).

new x265 API:
     const x265_api* api = x265_api_get(int bitDepth);
     x265_api - holds the libx265 public API functions
     bitDepth - requested API for 8bpp or 16bpp

     note: Use 0 to indicate native bit depth of the linked libx265 and
           x265_api_get(0) is guaranteed to return a non-null pointer
Signed-off-by: Gopu Govindaswamy <gopu@multicorewareinc.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>