[windows] OpenCV VideoCapture SEH exception with some backends

[funnbot]

From my testing the obs-virtualcam dll that gets runtime loaded can throw SEH exceptions which cannot be caught, although considering how frequently people experience this, its likely more backends can aswell.

The only way to catch SEH exceptions in C++ is to compile with the /EHa option, while /EHsc is the default for most C++ apps, as /EHa can introduce some performance and binary size issues.

This leaves the alternative of spawning a separate process to test the camera address, before the main process does.
A small exe distributed in the utilities folder will be included on windows builds, it only needs to link with OpenCV, and will attempt to open the hardware index, Using CreateProcess,the main process will then read the exit code and notify the user that the camera can't be opened with this address.