[imgui] Fixes to work with new api.

- Font atlas is now owned by context.
- Switch from deprecated io.RenderDrawListsFn callback to dedicated call in window.cc.
- Replaced deprecated ImGuiCol_ModalWindowDarkening with ImGuiCol_ModalWindowDimBg.
- Replaced deprecated SetScrollHere() with SetScrollHereY().
- Replaced deprecated GetContentRegionAvailWidth() with GetContentRegionAvail().x.
- Replaced deprecated ShowTestWindow() with ShowDemoWindow().
- Replaced deprecated ImGuiCol_ChildWindowBg with ImGuiCol_ChildBg.
- Replaced deprecated SetNextTreeNodeOpen() with SetNextItemOpen().

src/xenia/ui/imgui_drawer.h

@@ -17,7 +17,6 @@
 #include "xenia/ui/window_listener.h"
 
 struct ImDrawData;
-struct ImFontAtlas;
 struct ImGuiContext;
 struct ImGuiIO;
 
@@ -35,6 +34,7 @@ class ImGuiDrawer : public WindowListener {
   void SetupDefaultInput() {}
 
   ImGuiIO& GetIO();
+  void RenderDrawLists();
 
   static const uint64_t kIgnoreAlpha = (1ull << 63);
 
@@ -58,7 +58,6 @@ class ImGuiDrawer : public WindowListener {
   GraphicsContext* graphics_context_ = nullptr;
 
   ImGuiContext* internal_state_ = nullptr;
-  std::unique_ptr<ImFontAtlas> font_atlas_;
   std::unique_ptr<ImmediateTexture> font_texture_;
 };