On iOS, how do I explicitly download a photo or video that's in iCloud while using iCloud Photo Library? I'm an iCloud Photo Library user.  How can I specifically ensure that a specific video/photo is downloaded locally on my iOS device?

Basically I'm looking for the equivalent of a "Download" button available on a photo or video so that it will be available locally on the iPhone.

Basically, there are some apps that don't really handle deal with iCloud based media well, so I want to ensure that the media is available locally so I can accomplish my task in the 3rd party app (Magisto).
If your device is not set to Optimize Storage, it will not remove a download, meaning you can then go open the photo in Photos and it will be there permanently.

Additionally, if it's a short time frame this will work even with Optimize Storage.