Peek flat shulker icons

This doesn’t add much in the way of new block support, just plain decorated pots and skulls, but it does have quite a few changes I wanted to make before it was good enough for calling release rather than beta.

• Adds decorated pot base model
• Adds player and mob heads
• Walls are now constructed from block textures
• Fences are now constructed from block textures
• Changes suspicious blocks to use overlay textures
• Fixed incorrect rendering for shields and the debug stick.