Blit API functions and properties are used when combining buffers.
From Blit API functions, the connection to screen should have been acquired with the function screen_create_context().
Any source and destination buffers required by the Blit API functions are specified with buffer handles. These handles are typcially acquired by querying the SCREEN_PROPERTY_RENDER_BUFFERS property of a pixmap or window with the screen_get_pixmap_property() or screen_get_window_property() functions respectively.