Issue title: Zoom support
 Issue body: Hi, I use Vysor on a daily basis and I was wondering if it's possible to zoom in and out?
 Comments: 
 Comment 0: I want to add a voice to this request. I purchased the app and use it almost daily, but the lack of ability to pinch-to-zoom is driving me crazy!
 Comment 1: third this.
 Comment 2: For the people how read this because they can't zoom:
Alt + left click allow you to enter multitouch (you can now zoom by pinch)
Alt + left click again to disable multitouch
 Comment 3: > For the people how read this because they can't zoom:
> Alt + left click allow you to enter multitouch (you can now zoom by pinch)
> Alt + left click again to disable multitouch

Can I get some clarification on that? When I Alt + left click, i get a red dot... now what? 

Can we just get some simple pinch zoom in/out like by holding alt and scrolling the mouse wheel?
 Comment 4: This is not a proper solution. If the developer thinks it is, then more information needs to be provided on how this is supposed to be used. Pressing Alt + left click only leaves a red target, and I have no idea how to make the Zoom function work. I have paid for Vysor, so I feel I am in my right to request further information and support on this topic.
 Comment 5: use the alt click to "plant" a finger.
use the mouse to simulate movement with a second finger.
 Comment 6: > use the alt click to "plant" a finger. use the mouse to simulate movement with a second finger.

Unfortunately absolutely nothing happens when I move the mouse after planting the red dot "finger".
 Comment 7: > use the alt click to "plant" a finger. use the mouse to simulate movement with a second finger.

Just like @redreinard, this does nothing for me.
{'source': 'GithubIssues', 'repo_name_with_owner': 'koush/vysor.io', 'issue_id': 320479212, 'repo_stars': 1287.0}