Added float GetWindowDpiScale(). (#1542, #1676)

TODO.txt

@@ -264,6 +264,8 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - focus: SetKeyboardFocusHere() on with >= 0 offset could be done on same frame (else latch and modulate on beginning of next frame)
  - focus: unable to use SetKeyboardFocusHere() on clipped widgets. (#787)
 
+ - examples: move ImGui::NewFrame() out of the backend _NewFrame() ?
+ - viewport: make it possible to have no main/hosting viewport
  - viewport: use getfocus/setfocus api to synchronize imgui<>platform focus better (e.g imgui-side ctrl-tab can focus os window, OS initial setup and alt-tab can focus imgui window etc.)
  - viewport: store per-viewport/monitor DPI in .ini file so an application reload or main window changing DPI on reload can be properly patched for.
  - viewport: vulkan renderer implementation. 

imgui.cpp

@@ -8192,11 +8192,18 @@ ImDrawList* ImGui::GetWindowDrawList()
     return window->DrawList;
 }
 
+float ImGui::GetWindowDpiScale()
+{
+    ImGuiContext& g = *GImGui;
+    IM_ASSERT(g.CurrentViewport != NULL);
+    return g.CurrentViewport->DpiScale;
+}
+
 ImGuiViewport* ImGui::GetWindowViewport()
 {
-    ImGuiWindow* window = GetCurrentWindowRead();
-    IM_ASSERT(window->Viewport != NULL);
-    return window->Viewport;
+    ImGuiContext& g = *GImGui;
+    IM_ASSERT(g.CurrentViewport != NULL && g.CurrentViewport == g.CurrentWindow->Viewport);
+    return g.CurrentViewport;
 }
 
 ImFont* ImGui::GetFont()

imgui.h

@@ -197,6 +197,7 @@ namespace ImGui
     IMGUI_API bool          IsWindowFocused(ImGuiFocusedFlags flags=0); // is current window focused? or its root/child, depending on flags. see flags for options.
     IMGUI_API bool          IsWindowHovered(ImGuiHoveredFlags flags=0); // is current window hovered (and typically: not blocked by a popup/modal)? see flags for options. NB: If you are trying to check whether your mouse should be dispatched to imgui or to your app, you should use the 'io.WantCaptureMouse' boolean for that! Please read the FAQ!
     IMGUI_API ImDrawList*   GetWindowDrawList();                        // get draw list associated to the current window, to append your own drawing primitives
+    IMGUI_API float         GetWindowDpiScale();                        // get DPI scale currently associated to the current window's viewport.
     IMGUI_API ImGuiViewport*GetWindowViewport();                        // get viewport currently associated to the current window.
     IMGUI_API ImVec2        GetWindowPos();                             // get current window position in screen space (useful if you want to do your own drawing via the DrawList API)
     IMGUI_API ImVec2        GetWindowSize();                            // get current window size