Gets the true monitor resolution of the input source for multi-monitor setups

[emptycrown]

Something like this is needed to actually use the true monitor resolution to scale a scene. When there is only one monitor, this doesn't matter much since Electron's getPrimaryDisplay() will handle that.

However, in multi-monitor setups, Electron's displays are not helpful. To get the resolution dimensions, we must wait until after the source is added and then scale the scene accordingly.

Solves #15