Allow to render with Cairo
Rendering the video with OpenGL have been introduced recently, enabling advanced video filters and in theory speeding up the rendering. Unfortunately some users experience slowdowns since the introduction of RetroGLDisplay (see #4).
To help them until we fix the performance problem, we could allow to use the Cairo-based display with, triggering it with an environment variable similar to WebKitGTK+'s WEBKIT_DISABLE_COMPOSITING_MODE: https://trac.webkit.org/wiki/EnvironmentVariables.