Ensure that always one shader is on the ShaderStack with GL2
This brings some consistency with GLES2 where also one shader
is always on the stack and allows to use the test whether a shader
is bound to see if the GL2 code path is used. This fixes an issue
with cover/flip switch which tested for a bound shader before the
shader was pushed in multi screen setups.
As well it should bring some improvements as the simple screen
shader is not unbound and rebound in each rendered frame.