[viewer] add FPS counter using imgui

[msbaines]

Motivation and Context

Would be cool to be able to use imgui for HeadsUpDisplay

How Has This Been Tested

Ran viewer and verified that FPS counter is displayed correctly.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[mosra]

Plain and simple. Thank you! :)

[bigbike]

I was thinking "iamgui" might be heavy in terms of development but clearly I am wrong! I sincerely appreciate your effort to bring it to us during the weekend.

I remember when I initialized #343, I directly applied a couple of Magnum libraries (fontManager, fonts, Renderer2D) to the "viewer", the reviewers required me to extract the "text drawing" in a separate class to keep the viewer clean and code re-usable. I hope "iamgui" would serve this purpose and address their concerns even though there is certain "iamgui" related logic exposed in the code (e.g., setBlendFunction, ImGui::Begin, SetWindowFontScale(...)). Hope they can tolerate it.

Thank you very much, @msbaines!

About implementation details:
I am not sure if it is possible to move the "text display" to the TOP-LEFT or TOP-RIGHT? That would make it perfect.

Also please make sure it can be compiled in --headless mode as well.

[bigbike]

Also, would you consider adding a boolean switch to turn on/off the info display?
Check the displayPerformanceInfo in #343.