Add mask support to floodFill

[basvandijk]

In #20 @timjb mentioned the following issue regarding the floodFill function:

Mask matrix support is still missing. I propose adding an argument of the following type to floodFill:

data FloodFillMask shape
   = FloodFillNoMask
   | FloodFillMask (MutMat (TwoLarger shape) ('S 1) Word8 (PrimState m))

type family TwoLarger :: DS [DS Nat] -> DS [DS Nat] where
    TwoLarger 'D = 'D
    TwoLarger ('S '[w, h]) = 'S [PlusTwo w, PlusTwo h]

type family PlusTwo :: DS Nat -> DS Nat where
    PlusTwo 'D = 'D
    PlusTwo ('S n) = 'S (n + 2)

Another issue: When using the FLOODFILL_MASK_ONLY flag, the image matrix can be immutable, since only the mask is written (and we don't need to ask the user for a fill color in this case). Maybe there should be a separate function for that usecase.