Tetrogue 1.3.1 released

Better to publish small changes right away

• Fix lightning and bomb sound: 
  • Only one copy of the sound plays at a time. (Avoiding an ear-splitting cacophony when slamming down 4+ lightning blocks)
  • The full lightning sound plays, rather than just the first split second of it. (Maybe the cut-off version sounded better. Tell me and I'll bring it back.)

• Update anvil and gift sprite cause I felt like drawing.
• Misc sprite updates to remove all transparency in blocks, making them nice and square. (Except for lightning)
• Fix piece shadows so they match the updates sprite textures.