UnboundLocalError in detect_scenes when frame_skip is larger than 0.

[twostarxx]

Bug/Issue Description:

"UnboundLocalError: local variable 'read_frame' referenced before assignment" when running scene_manager.detect_scenes(frame_source=video_manager, frame_skip=1)

Required Information:

We hit this error when frame_skip is larger than 0.

Expected Behavior:

It works when we assign frame_skip.

Computing Environment:

• OS: [e.g. Windows, Linux (Distro: Ubuntu, Mint, Fedora, etc...), OSX]
• Python Version: [e.g. 3.6 or 3.6.6]
• OpenCV Version: [e.g. 3.4.1]

Additional Information:

[Breakthrough]

Thank you for your issue report and pull request!

This will be included in the next release of PySceneDetect, thank you so much.