I'm seeing a noticable degradation in quality when rendering a larger image into a smaller bitmap context. I've confirmed it's new to Mojave, I think it just started in Beta 5 but I might be wrong. The same rendering operation on 10.13 yields a higher quality image than the operation running on 10.14. It doesn't seem linked to SDK linkage version or deployment target.
I filed Radar #42777290 with a sample project that reproduces the problem, in case anybody within Apple is curious to test it.
Hopefully this will get fixed before the GM!