`drag` doesn't track "start of drag" as distinct from "mid drag"

[sboosali]

@minimapletinytools seems to have implemented this in tinytools-vty:

data DragState = DragStart | Dragging | DragEnd

data Drag2 = Drag2
  { …
  , _drag2_state :: DragState
  }

drag2 =
  …
  V.EvMouseDown x y btn' mods'
          | state == DragEnd && btn == btn' -> Just $ Drag2 (x,y) (x,y) btn' mods' DragStart
          | state /= DragEnd && btn == btn' -> Just $ Drag2 from (x,y) btn mods' Dragging
          | otherwise
  …

https://github.com/minimapletinytools/tinytools-vty/blob/potato/src/Potato/Reflex/Vty/Widget.hs#L183