bitmap: Store a pointer to the context

This adds a context member to CoglBitmap which stores the context it
was created with. That way it can be used in texture constructors
which use a bitmap. There is also an internal private function to get
the context out of the bitmap which all of the texture constructors
now use. _cogl_texture_3d_new_from_bitmap has had its context
parameter removed so that it more closely matches the other bitmap
constructors.

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

cogl/cogl-bitmap-private.h

@@ -142,4 +142,7 @@ _cogl_bitmap_bind (CoglBitmap *bitmap,
 void
 _cogl_bitmap_unbind (CoglBitmap *bitmap);
 
+CoglContext *
+_cogl_bitmap_get_context (CoglBitmap *bitmap);
+
 #endif /* __COGL_BITMAP_H */