FrameTimeDiagnosticsPlugin should track real frame time when vsync is enabled

## What problem does this solve or what need does it fill?

I'd like to know how close I am to my budget by looking at the frame time.

## What solution would you like?

When vsync is enabled, the frame time should not include the time spent waiting to draw the next frame.

## What alternative(s) have you considered?

Disable vsync to get my actual frame times.