Fix large file uploads producing oversized packets.

- Fixes #1026
- The remaining bytes counter wasn't being decremented, so the code that
  splits off smaller packets was unreachable. Thus all file slices were
  being put into a single UploadFileMessage packet.

src/main/java/dan200/computercraft/shared/network/server/UploadFileMessage.java

@@ -160,6 +160,7 @@ public class UploadFileMessage extends ComputerServerMessage
                 contents.position( currentOffset ).limit( currentOffset + canWrite );
                 slices.add( new FileSlice( fileId, currentOffset, contents.slice() ) );
                 currentOffset += canWrite;
+                remaining -= canWrite;
             }
 
             contents.position( 0 ).limit( capacity );