VSE: Render in size nearest to preview image

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:
{F9422536}

Differential Revision: https://developer.blender.org/D9414