DEV-42989: Adjust media priority based on app minimization and focus

Review #49

This change adjusts each media's priority based on whether the viewer is minimized (media priority becomes HIDDEN) or unfocused (media priority becomes LOW). However, due to the fact that updateMedia() was no longer being called when minimized, I moved its call out of LLViewerTextureList::updateImages() (a seemingly odd place anyway) and into its own idle callback.