[wpigui] Limit frame rate to 120 fps by default (#5030)

Limiting with vsync is apparently unreliable on a number of systems;
this resulted in high CPU/GPU usage.

Also add current actual frame rate to about dialog of GUI tools.

glass/src/app/native/cpp/main.cpp

@@ -269,6 +269,8 @@ int main(int argc, char** argv) {
       ImGui::Text("v%s", GetWPILibVersion());
       ImGui::Separator();
       ImGui::Text("Save location: %s", glass::GetStorageDir().c_str());
+      ImGui::Text("%.3f ms/frame (%.1f FPS)",
+                  1000.0f / ImGui::GetIO().Framerate, ImGui::GetIO().Framerate);
       if (ImGui::Button("Close")) {
         ImGui::CloseCurrentPopup();
       }