Added more accurate GPU Timing

[RuffledPlume]

Problem:

From doing GPU Captures & Traces, I've noticed that adding the GPU Markers together doesn't paint the full picture of how long a frame is being drawn. GPU Idle time,

Solution: Added new Timer: RENDER_FRAME, to capture the time between PostDrawScene() & the Submit in Draw().

Caveat: This does mean it'll capture the GPU Idle time between the compute & submit, but overall I've found this to be more accurate when testing changes.

[RuffledPlume]

Noticed that both checkGlError() & rendering the FrameTimerOverlay were adding almost a whole millisecond to the CPU timer, both of these are only ever opened in developer builds, so we should try to remove them whilst the UI is opened