cogl: Implement retained clip stacks

This adds three new internal API functions which can be used to retain
the clip stack state and restore it later:

 _cogl_get_clip_stack
 _cogl_set_clip_stack
 _cogl_clip_stack_copy

The functions are currently internal and not yet used but we may want
to make them public in future to replace the cogl_clip_stack_save()
and cogl_clip_stack_restore() APIs.

The get function just returns the handle to the clip stack at the top
of the stack of stacks and the set function just replaces it.

The copy function makes a cheap copy of an existing stack by taking a
reference to the top stack entry. This ends up working like a deep
copy because there is no way to modify entries of a stack but it
doesn't actually copy the data.

1 files changed, 26 insertions, 0 deletions

diff --git a/cogl/cogl-clip-stack.c b/cogl/cogl-clip-stack.c
index 37e7d398..63221561 100644
--- a/cogl/cogl-clip-stack.c
+++ b/cogl/cogl-clip-stack.c
@@ -682,3 +682,29 @@ _cogl_clip_stack_free (CoglClipStack *stack)
 
   g_slice_free (CoglClipStack, stack);
 }
+
+CoglHandle
+_cogl_clip_stack_copy (CoglHandle handle)
+{
+  CoglHandle new_handle;
+  CoglClipStack *new_stack, *old_stack;
+
+  if (!cogl_is_clip_stack (handle))
+    return COGL_INVALID_HANDLE;
+
+  old_stack = COGL_CLIP_STACK (handle);
+
+  new_handle = _cogl_clip_stack_new ();
+  new_stack = COGL_CLIP_STACK (new_handle);
+
+  /* We can copy the stack by just referencing the other stack's
+     data. There's no need to implement copy-on-write because the
+     entries of the stack can't be modified. If the other stack pops
+     some entries off they will still be kept alive because this stack
+     holds a reference. */
+  new_stack->stack_top = old_stack->stack_top;
+  if (new_stack->stack_top)
+    new_stack->stack_top->ref_count++;
+
+  return new_handle;
+}