If somebody decides to use gtk_widget_set_double_buffered() in the
middle of a draw() then there's the risk of calling end_draw_frame()
with an invalid pointer.

Some overeager compilers may warn about the double_buffered bit field
changing values and leading to a potentially uninitialized variable.

In order to avoid compiler warnings or crashes, we can simply store the
value of the double_buffered bit field at the beginning of the rendering
and use that instead of the actual bit field.

https://bugzilla.gnome.org/show_bug.cgi?id=771463