Calculate nearest power of two render size based on image size in
preview area when "Proxy Render Size" is set to "Automatic".
Downscaling is implemented for movies on IO level - see IMB_Downscale
and IMB_anim_absolute(). Movies are also downscaled by factor which
is power of two. Currently up to 32x - IMB_DOWNSCALE_32X
This means that with smaller preview sizes optimal image sizes are used
resulting in lower memory usage and higher performance.
Demo 8K video preview performance + debug info how it works internally:

