Returns the current payload size the given frame have without taking into account mime headers.
This function will return current payload size stored on the given frame, skipping mime header size. This is because Vortex Library manages mime headers for every frame received, making a separation between the mime header part and the mime content.
However, if you don't use mime headers, this function will return the entire payload size.
If you need to get current frame content size, including payload size and mime content size, use vortex_frame_get_content_size.