Alpha-channel shadow support

[FraserLee]

There's an open bevy issue for this at bevyengine/bevy#4372, seems like the most straight-forward path would be to go in and resolve this in the engine. Barring that, there are a few more patch-style fixes we could try:

• Manually creating a far simpler interim shader for sprites, possibly at the cost of PBR
• Analytically modifying mesh shape to fit the contours of a non-transparent sprite
  • This might have some side-benefits in collision, mouse picking, etc.

Will look into this more later.

[FraserLee]

bevy-0.10 fixes this. Nice.