Follow-up from "Enumerate Mesa DRI modules and VA-API drivers"

Spun off from !84 (merged):

It would be nice if we could have

const char *srt_va_api_driver_get_library_path (SrtVaApiDriver *);

typedef enum
{
  SRT_VA_API_VERSION_UNKNOWN = 0,
  SRT_VA_API_VERSION_LIBVA1 = 1,
  SRT_VA_API_VERSION_LIBVA2 = 2
} SrtVaApiVersion;

SrtVaApiVersion srt_va_api_driver_get_version (SrtVaApiDriver *);