For those playing the original myst series, we've found the following settings for the INI files for both Myst Masterpiece Edition and Riven, greatly improve the graphics quality at full screen. Hope this is helpful!

[scummvm]
gui_saveload_chooser=list
filtering=true
gfx_mode=OpenGL
fullscreen=true
console=false
render_mode=VGA
stretch_mode=Stretch
aspect_ratio=false

Thanks.
Are these the same settings the GoG version uses as well?

This doesn't "improve" the quality, it distorts the image.