Support StartFrameCapture(NULL) and EndFrameCapture(NULL). Refs #117

* If there's only one device/equivalent around, then allow specifying a
  NULL window for frame capturing and that lone device is the one used.
* This allows for capturing completely headless applications, perhaps doing
  compute or something offline. It also allows capturing applications where
  the rendering happens by sharing textures off to some other API that
  isn't hooked.