This new scale filter computes pixels by weighing the coverage area of
source pixels over the target pixel. This algorithm works well for both
upsampling and downsampling, but was mainly designed to upscale
high-quality low-resolution sources like RGB/HDMI retro consoles. I've
heard of people using odd workarounds like scaling up to very high
resolutions before scaling back down to preserve pixel shartpness. This
algorithm directly addresses this use-case in a much more direct
fashion.

The Area scale filter does a better job of preserving the thickness of
thin features than the Point filter.

The Area scale filter does not look at source pixels that lie outside
of the target pixel, leading to a much sharper image than Bilinear,
Bicubic, and Lanczos filters.

This filter should interpolate pixels in linear space, but OBS is not
equipped to do that at the moment.

libobs: Add GPU effect, and wire up scene serialization.

obs-filters: Add Area as an option for scale_filter.

UI: Add Area as an option for both scene items, and canvas downscaling.