Viewports: used main viewport for centering (wip), clarified the meaning of how ImGuiPlatformMonitor WorkPos/WorkSize should be set if unknown, added asserts.

imgui.h

@@ -2570,7 +2570,7 @@ struct ImGuiPlatformIO
 struct ImGuiPlatformMonitor
 {
     ImVec2  MainPos, MainSize;      // Coordinates of the area displayed on this monitor (Min = upper left, Max = bottom right)
-    ImVec2  WorkPos, WorkSize;      // (Optional) Coordinates without task bars / side bars / menu bars. imgui uses this to avoid positioning popups/tooltips inside this region.
+    ImVec2  WorkPos, WorkSize;      // Coordinates without task bars / side bars / menu bars. Used to avoid positioning popups/tooltips inside this region. If you don't have this info, please copy the value for MainPos/MainSize. 
     float   DpiScale;               // 1.0f = 96 DPI
     ImGuiPlatformMonitor()          { MainPos = MainSize = WorkPos = WorkSize = ImVec2(0, 0); DpiScale = 1.0f; }
 };
@@ -2621,7 +2621,8 @@ struct ImGuiViewport
     ImGuiViewport()     { ID = 0; Flags = 0; DpiScale = 0.0f; DrawData = NULL; ParentViewportId = 0; RendererUserData = PlatformUserData = PlatformHandle = PlatformHandleRaw = NULL; PlatformRequestMove = PlatformRequestResize = PlatformRequestClose = false; }
     ~ImGuiViewport()    { IM_ASSERT(PlatformUserData == NULL && RendererUserData == NULL); }
 
-    // Access work-area rectangle
+    // Access work-area rectangle with GetWorkXXX functions (see comments above)
+    ImVec2              GetCenter()             { return ImVec2(Pos.x + Size.x * 0.5f, Pos.y + Size.y * 0.5f); }
     ImVec2              GetWorkPos()            { return ImVec2(Pos.x + WorkOffsetMin.x, Pos.y + WorkOffsetMin.y); }
     ImVec2              GetWorkSize()           { return ImVec2(Size.x - WorkOffsetMin.x + WorkOffsetMax.x, Size.y - WorkOffsetMin.y + WorkOffsetMax.y); } // This not clamped
 };