[feature request] show performance graph when all hope is lost

[ThomasSteve83]

idea for an optional feature: show steamvr gpu performance graph in headset when not meeting framerate/frametime target and the resolution can't be decreased, such as:

• when using manual resolution
• when at minimum resolution
• when running a blacklisted app

this will highlight performance issues that can't be solved, encouraging users to change game graphics settings and such

shouldn't be on by default, most users don't want to have performance issues pointed out, but would be useful for testing and fine-tuning settings