NvFBC is unable to capture certain applications that bypasses the DWM
compositor, for example, Firefox playing video in full screen. This
has been a known issue for a long time with Nvidia's ShadowPlay, see:
* https://www.nvidia.com/en-us/geforce/forums/geforce-experience/14/233709/
* https://crbug.com/609857
Nvidia won't fix this, but there are workarounds. For example, we
create a transparent 1x1 layered window, which forces desktop composition
to be enabled.
Note that SetLayeredWindowAttributes also supports alpha-based transparency,
but setting transparency to 0 will cause DWM to skip composition. We could
use a transparency of 1, but this ruins the image by the slightest bit,
which is unacceptable. Therefore, we must use chroma key-based
transparency, which tricks DWM into compositing despite being fully
transparent.
