Improve readability & controls of sprite depth example

[pushfoo]

Follow-up to reading #1729 (comment)

Changes

• Move sprite depth demo to a clearer file name
• Add a top-level docstring including run instructions per arcade's conventions
• Move hard-coded values into adjustable constants
• Add space bar toggle for depth testing
• Add on-screen instruction / status display using batched text matching other examples
• Add comments

How to test

python -m arcade.experimental.sprite_depth_cosine

[einarf]

👍