Fixed wrong camera selection for Surface tablets

[AgentRev]

On Microsoft Surface tablets, Prey takes pictures with the rear camera by default. This isn't really practical, as it is unlikely to provide relevant information.

The cameras on Surface tablets are identified as "Microsoft Camera Front" and "Microsoft Camera Rear", therefore adding rear to the -invalid parameter for prey-webcam.exe forces Prey to use the front camera, as is intended.

Successfully tested on Surface 3, should in theory also work for Surface Pro 3, Surface Pro 4, Surface Book, and possibly older models as well.

Related: #98

[mauricioschneider]

Great finding! I'll test it and include it in release 1.5.1.

[AgentRev]

Tested working on Surface Pro 2017.