Bug 1229125 - Correct velocity computation for pan gesture events. r=botond

When scrolling down, panInput.mLocalDisplacement.y will be positive, so
aAdditionalDelta will be positive, and newVelocity must be positive.

--HG--
extra : commitid : GlT0IqOWqiI
extra : rebase_source : 2fc0347d943ac7bf8eac2f2da6f7b6b217bdbe0d

gfx/layers/apz/src/Axis.cpp

@@ -76,7 +76,7 @@ void Axis::UpdateWithTouchAtDevicePoint(ParentLayerCoord aPos, ParentLayerCoord
     return;
   }
 
-  float newVelocity = mAxisLocked ? 0.0f : (float)(mPos - aPos - aAdditionalDelta) / (float)(aTimestampMs - mPosTimeMs);
+  float newVelocity = mAxisLocked ? 0.0f : (float)(mPos - aPos + aAdditionalDelta) / (float)(aTimestampMs - mPosTimeMs);
   if (gfxPrefs::APZMaxVelocity() > 0.0f) {
     bool velocityIsNegative = (newVelocity < 0);
     newVelocity = fabs(newVelocity);