allow keeping maya's cachedPlayback Cache

[vincentullmann]

Changelog Description

Refactor the _applied_camera_options method to only apply and revert those attributes where the values differ from the current ones.
The generation of thumbnail would use the current frame instead of the start frame.

Additional review information

This allows maya to keep the "cachedPlayback" Cache and thus can impove playblast performance

Testing notes:

1. open a heavy maya scene (eg.: large image planes)
2. scrub trough the timeline to populate the playback cache
3. make sure your camera settings match the ExtractPlayblast settings!
4. publish a playblast