Use bytearray instead of BytesIO in MspDecoder

[radarhere]

1. When iterating through rows, the row index is assigned to x
   

   Pillow/src/PIL/MspImagePlugin.py

   Line 129 in 3220aca

   for x, rowlen in enumerate(rowmap):

   
   y would seem to be the correct choice instead.

2. MspDecoder writes to a io.BytesIO instance, only to retrieve the value from it once done. Using bytearray() would be simpler.