Remove the framebuffer's stack of clip stacks

There used to be a function called cogl_clip_stack_save in the public
API which was used when temporarily switching to an offscreen buffer
to save the clip state. This is no longer necessary because each
framebuffer has its own clip stack anyway so the function was removed
in master. However the code to maintain the stack of stacks was
retained. This patch removes it in an effort to simplify the code.

On the 1.18 branch this function is deprecated and the documentation
says that it does nothing. However that is incorrect because it does
actually the push clip stack. I think it would be safe to backport
this patch to the 1.18 branch and actually make it do nothing like it
is documented to do.

https://bugzilla.gnome.org/show_bug.cgi?id=719546

Reviewed-by: Robert Bragg <robert@linux.intel.com>