Resolve native libraries at runtime rather than at compile time

[alextnull]

Not tested on Linux and Unity.

Changes
Resolve native libraries at runtime rather than at compile time.
Single file for calling native library.
Fix encoder and decoder dispose.

Breaking changes
Defines.CiscoDllName64bit deprecated. Library name must be passed to the decoder or encoder constructor.

// Example
string ciscoDll = "";
switch (RuntimeScanApi.OperatingSystem)
{
    case H264Sharp.OperatingSystem.Windows:
        ciscoDll = RuntimeScanApi.Is64BitProcess
            ? "openh264-2.4.0-win64.dll"
            : "openh264-2.4.0-win32.dll";
        break;
    case H264Sharp.OperatingSystem.Linux:
        ciscoDll = RuntimeScanApi.Is64BitProcess
            ? "libopenh264-2.4.1-linux64.7.so"
            : "libopenh264-2.4.1-linux32.7.so";
        break;
    default:
        throw new NotSupportedException($"{RuntimeScanApi.OperatingSystem} OS is not supported");
}
H264Encoder encoder = new H264Encoder(ciscoDll);
H264Decoder decoder = new H264Decoder(ciscoDll);