Make kinetic and two-finger scrolling appear smoother
We'll still render the same number of frames, but this patch allows us to draw on half-unit boundaries which may still map to a physical pixel. Doing so improves the user-perception of smoothness/performance because text doesn't jump two (or three on 3x HiDPI) physical pixels for each scroll step.
For what it's worth, I have patches to do the same thing inside of GtkTextView but can't land them until GTK 5.x due to ABI of GtkTextView. Thankfully here that is not an issue.