curlhttpsrc: add support for range GET
To allow curlhttpsrc to support DASH streams that use the on-demand profile, it needs to support HTTP Range GETs. In GStreamer, the RANGE is specified by issuing a GST_FORMAT_BYTES seek to set the start and end of the range. curlhttpsrc needs to implement seek and set the appropriate curl options to make it add the Range header to the request.