Render tests: capture all channels images for the same frame

[w23]

Currently render tests advance for the next frame for each channel, so that different channel images are not taken for the same frame. This doesn't help with determinism, as frame deltas vary, subtly affecting various things.
Why it is done so: rendertests are added cheaply on top of debug single-channel display and taking screenshots.

Ideally, there'd be a special command that'd capture all channels at once and store them together.

It'd be also great to make engine more deterministic, at least synchronise with the first frame, and/or provide fake fixed framerate for the first few frames (for the denoiser to collect more statistics).
That being said we'd probably benefit from having rendertests for

• first frame (raw, not temporally denoised)
• Nth frame (temporally denoised)