dr_dscan
dr_dscan = drdy + dxdy * drdx
fdrOuter = drdy + dxOuter * drdx
fdrInner = fdrOuter + fdrdx (Inner differs from Outer by one pixel).
For this bottom subtriangle, drdy = 0.
Mesa computes dxOuter as:
dxOuter = Integer (dxdy) (drop fractional part)
Since dxdy = 0.685, dxOuter = 0, so fdrOuter = 0, fdrInner = fdrdx.
End Result: If Xleft does not change, then Rs does not change!
Rs tied to integer changes in X, Y. Term drdy already has an implied one scan line change in it.