Declared non-seekable streams as non-seekable in callback input

These are the same changes as ZeBobo5/Vlc.DotNet#648

Also increased the buffer size to match the libvlc buffer size.

# Conflicts:
#	src/LibVLCSharp/Media.cs

src/LibVLCSharp/Media.cs

@@ -41,7 +41,7 @@ internal struct Native
             [DllImport(Constants.LibraryName, CallingConvention = CallingConvention.Cdecl,
                 EntryPoint = "libvlc_media_new_callbacks")]
             internal static extern IntPtr LibVLCMediaNewCallbacks(IntPtr libVLC, InternalOpenMedia openCb, InternalReadMedia readCb,
-                InternalSeekMedia seekCb, InternalCloseMedia closeCb, IntPtr opaque);
+                InternalSeekMedia? seekCb, InternalCloseMedia closeCb, IntPtr opaque);
 
             [DllImport(Constants.LibraryName, CallingConvention = CallingConvention.Cdecl,
                 EntryPoint = "libvlc_media_add_option")]
@@ -295,7 +295,7 @@ static IntPtr CtorFromInput(LibVLC libVLC, MediaInput input)
             return Native.LibVLCMediaNewCallbacks(libVLC.NativeReference,
                 OpenMediaCallbackHandle,
                 ReadMediaCallbackHandle,
-                SeekMediaCallbackHandle,
+                input.CanSeek ? SeekMediaCallbackHandle : null,
                 CloseMediaCallbackHandle,
                 GCHandle.ToIntPtr(input.GcHandle));
         }

src/LibVLCSharp/MediaInput.cs

@@ -23,6 +23,11 @@ protected MediaInput()
             GcHandle = GCHandle.Alloc(this);
         }
 
+        /// <summary>
+        /// A value indicating whether this Media input can be seeked in.
+        /// </summary>
+        public bool CanSeek { get; protected set; } = true;
+
         /// <summary>
         /// LibVLC calls this method when it wants to open the media
         /// </summary>

src/LibVLCSharp/StreamMediaInput.cs

@@ -21,6 +21,7 @@ public class StreamMediaInput : MediaInput
         public StreamMediaInput(Stream stream)
         {
             _stream = stream ?? throw new ArgumentNullException(nameof(stream));
+            CanSeek = stream.CanSeek;
         }
 
         /// <summary>