Windows, Internal: added experimental SkipRefresh mode. (#3515, #4763, #7556, #5116 , #4076, #2749, #2268)

currently: ImGui::SetNextWindowRefreshPolicy(ImGuiWindowRefreshFlags_TryToAvoidRefresh);
- This is NOT meant to replace frame-wide/app-wide idle mode.
- This is another tool: the idea that a given window could avoid refresh and reuse last frame contents.
- I think it needs to be backed by a careful and smart design overall (refresh policy, load balancing, making it easy and obvious to user).
- It's not there yet, this is currently a toy for experimenting.
My other issues with this:
- It appears to be very simple, but skipping most of Begin() logic will inevitably lead to tricky/confusing bugs. Let's see how it goes.
- I don't like very much that this opens a door to varying inconsistencies
- I don't like very much that it can lead us to situation where the lazy refresh gets disabled in bulk due to some reason (e.g. resizing a dock space) and we get sucked in the temptation to update for idle rather than update for dynamism.

imgui_draw.cpp

@@ -385,6 +385,7 @@ void ImDrawListSharedData::SetCircleTessellationMaxError(float max_error)
 }
 
 // Initialize before use in a new frame. We always have a command ready in the buffer.
+// In the majority of cases, you would want to call PushClipRect() and PushTextureID() after this.
 void ImDrawList::_ResetForNewFrame()
 {
     // Verify that the ImDrawCmd fields we want to memcmp() are contiguous in memory.