Use the Multi-library interface to load at runtime x265 libraries
supporting alternative bit depths (e.g. 8bit and 16bit).

The linked library will try to load the library supporting the
pixel format if it is not supported by itself.

Fallback requesting the native library (passing 0 to x265_api_get) if
a library supporting the requested bit depth is not available.
Signed-off-by: Gopu Govindaswamy <gopu@multicorewareinc.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>