Wayland has gotten it right in the latest protocols but up to very recently it still did only integer scaling and scaled down for fractional. Scaling the buffer in fake coordinates is also needlessly complex. Just start witt the 150x90 buffer to begin with and the scaling is just a UI hint. All of that would work fine with X11 exactly because there's no need for fake coordinates. All scaling is the client's job based on a DPI or scale hint.