framebuffer: Make framebuffer_clear APIs public experimental

This makes cogl_framebuffer_clear and cogl_framebuffer_clear4f public as
experimental API. Since these functions take explicit framebuffer
pointers you don't need to push/pop a framebuffer just to clear it. Also
these functions are implicitly tied to a specific CoglContext via the
framebuffer pointer unlike cogl_clear.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-framebuffer-private.h

@@ -153,19 +153,6 @@ _cogl_framebuffer_clear_without_flush4f (CoglFramebuffer *framebuffer,
                                          float blue,
                                          float alpha);
 
-void
-_cogl_framebuffer_clear (CoglFramebuffer *framebuffer,
-                         unsigned long buffers,
-                         const CoglColor *color);
-
-void
-_cogl_framebuffer_clear4f (CoglFramebuffer *framebuffer,
-                           unsigned long buffers,
-                           float red,
-                           float green,
-                           float blue,
-                           float alpha);
-
 void
 _cogl_framebuffer_dirty (CoglFramebuffer *framebuffer);