[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/gpu/trace_viewer.cc

@@ -192,7 +192,7 @@ void TraceViewer::DrawMultilineString(const std::string& str) {
 }
 
 void TraceViewer::DrawUI() {
-  // ImGui::ShowTestWindow();
+  // ImGui::ShowDemoWindow();
 
   DrawControllerUI();
   DrawCommandListUI();
@@ -538,7 +538,7 @@ void TraceViewer::DrawCommandListUI() {
   }
   ImGui::PopID();
   if (did_seek && target_command == -1) {
-    ImGui::SetScrollHereY();
+    ImGui::SetScrollHereY(0.5f);
   }
 
   auto id = RecursiveDrawCommandBufferUI(frame, frame->command_tree.get());