resolution scaling for viewport/renderer, for fast preview renders

## Use Case

I would like to optimize render time by rendering fewer pixels into the same viewport

## Solution

* pixel sampling rates (1x1, 0.5x0.5, etc) - scale backbuffer and rescale to fit image resolution? (or repeat result of ray for e.g. 2x2 pixels) (alternative: splatting/dithering?)
* specific image resolution controls (and fit to viewport or not)
* allow python commands to script these
* enable fast preview modes that are very grainy but quick