Implement setSpeed in ofxiOSVideoPlayer

Fixes #2960

One thing that could use a second eye from @openframeworks/ios is the change from `[_player play]` to `[_player setRate:speed]` [here](https://github.com/admsyn/openFrameworks/blob/fix-ios-implement-video-setspeed/addons/ofxiOS/src/video/AVFoundationVideoPlayer.m#L679). Apple's docs say that "play" is "the same as setting rate to 1.0.", and the `speed` var was always initialized to 1, so this shouldn't break older projects.

This is necessary, since playback speed would get reset to 1.0 on looped playback otherwise.