Allow for sprite's Blend Mode to be updated programmatically

**Is your feature request related to a problem?**
I would like to change the Blend Mode of sprites programmatically.

Context: We have a lot of decals in our game, that we place using our custom level loader and I now have a "Decal" type, a "Screen Decal" type and a "Multiply Decal" type. Instead of a decal type with a Blend Mode property.

I can also imagine sprites changing Blend Mode based on game logics (e.g. Ghosts in Pacman changing appearance).

**Describe the solution you'd like:**
I would like to be able to change the Blend Mode of an instance of the sprite component of a game object:

```lua
msg.post(sprite_url, "blend_mode", sprite.BLEND_MULTIPLY)
```
or:
```lua
sprite.set_blend_mode(sprite_url, sprite.BLEND_SCREEN) -- like gui.set_blend_mode
```

Thanks!