Static Sprite Batching

[Syncaidius]

Add support for a static sprite-batching, which would avoid the cost of repopulating a sprite-batch buffer every time you want to draw large amount of sprites.

1. Split draw and batching methods of SpriteBatcher into a base class
2. Add StaticSpriteBatcher
3. Use a static vertex buffer instead of dynamic
4. Add StaticSpriteBatcher.Replace() for replacing existing sprites in the buffer without re-generating it all
5. Add StaticSpriteBatcher.ReplaceRange() for replacing a range of sprites at once
6. During Render() call:
   • If batch was changed, update vertex buffer via a staging buffer
   • Draw in the same way that normal SpriteBatcher does, but don't reset sprite data once finished
7. Add SpriteBatcher.FlushToStatic(StaticSpriteBatcher destination = null) to copy the current contents to a static sprite-batcher and flush without rendering