ConstReadonlyforce: 1Draws a frame even when nothing changed since the last one. It can't be
combined with FrameDemandFlag.waitForUpdate.
ReadonlywaitForUpdate: 4When the map has nothing new to draw, waits for its next update instead of
finishing with RenderResult.noUpdate.
How a frame demand behaves.
Without flags, a demand draws a frame when the map changed since the last one, and otherwise finishes with
RenderResult.noUpdate. A session with a surface target shows each frame that it draws.See
mln_frame_demand_flagin the C API reference.