libtorc-qml: Remove unused member variable in TorcSGVideoProvider
libtorc-qml: Minor OpenGL optimisation in TorcSGVideoProvider
libtorc-core: Add environment variable registration and help

- this lists Torc specific environment variables via the command line
help, which hopefully stops them getting lost and forgotten about.
libtorc-video: Add an environment variable override to disable VDA.
libtorc-qml: Add an environment variable override to disable VAAPI.
libtorc-qml: Add an enironment variable override to disable VDPAU