cogl: Move the clip stack dirtiness to the context rather than the FB

Previously we tracked whether the clip stack needs flushing as part of
the CoglClipState which is part of the CoglFramebuffer state. This is
a bit odd because most of the clipping state (such as the clip planes
and the scissor) are part of the GL context's state rather than the
framebuffer. We were marking the clip state on the framebuffer dirty
every time we change the framebuffer anyway so it seems to make more
sense to have the dirtiness be part of the global context.

Instead of a just a single boolean to record whether the state needs
flushing, the CoglContext now holds a reference to the clip stack that
was flushed. That way we can flush arbitrary stack states and if it
happens to be the same as the state already flushed then Cogl will do
nothing. This will be useful if we log the clip stack in the journal
because then we will need to flush unrelated clip stack states for
each batch.

cogl/cogl-context.h

@@ -187,6 +187,28 @@ typedef struct
      cogl_is_buffer */
   GSList           *buffer_types;
 
+  /* Clipping */
+  /* TRUE if we have a valid clipping stack flushed. In that case
+     current_clip_stack will describe what the current state is. If
+     this is FALSE then the current clip stack is completely unknown
+     so it will need to be reflushed. In that case current_clip_stack
+     doesn't need to be a valid pointer. We can't just use NULL in
+     current_clip_stack to mark a dirty state because NULL is a valid
+     stack (meaning no clipping) */
+  gboolean          current_clip_stack_valid;
+  /* The clip state that was flushed. This isn't intended to be used
+     as a stack to push and pop new entries. Instead the current stack
+     that the user wants is part of the framebuffer state. This is
+     just used to record the flush state so we can avoid flushing the
+     same state multiple times. When the clip state is flushed this
+     will hold a reference */
+  CoglClipStack    *current_clip_stack;
+  /* Whether the stencil buffer was used as part of the current clip
+     state. If TRUE then any further use of the stencil buffer (such
+     as for drawing paths) would need to be merged with the existing
+     stencil buffer */
+  gboolean          current_clip_stack_uses_stencil;
+
   CoglContextDriver drv;
   CoglContextWinsys winsys;
 } CoglContext;