936696afcf
Just like we do on EGL. Two bits are required because `cogl-clip-stack-gl.c` needs each stencil buffer element to be able to count from 0 to 2. This mistake probably went unnoticed because: * Drivers usually provide more than 1 anyway; and * Optimizations in `cogl-clip-stack-gl.c` avoid calling the code that needs to count past 1 in most cases. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1873>