Partial fix for EXT-4508 (nearby media floater parcel media button breaks rtsp)

This is actually two seperate issues:

1) After disabling parcel media from the nearby media floater, it can't be re-enabled
2) When you disable parcel media from the nearby media floater, faces displaying it don't revert to their original texture

This change fixes issue (1), by forcing the priority of the inworld media instance to normal in LLViewerMedia::updateMedia() so it will always get reloaded instead of possibly being kept unloaded due to already-loaded prim media.

Issue (2) is still under investigation.  In addition to the case where you disable parcel media from the nearby media floater, it also doesn't restore the textures when you leave the parcel, and I think I've even seen it when unloading