This fixes up some issues that were missed with the first PR in #1343

• Instead of checking for obs_source_active it now checks for obs_source_showing to ensure the filter is valid
  • This allows sending the video data when the source is showing in Studio mode for example)
• Removed the NDI sender create logic inside the ndi_filter_tick method
  • This was leftover from before the changes to the "timeout" were made when it destroyed the NDI sender when the source became invalid (e.g. hidden, or invalid size)
• Addition of a rendered boolean check
  • This ensures it will only render and send the source once, otherwise when it is visible multiple times (e.g. when the Properties window is opened) it will send frames out of order and it will look like it's stuttering (See video below)