Improve G.R.E.T.A. pixel art

[albrow]

Currently the pixel art for G.R.E.T.A. is a little plain and could use some more dirt, scratches, or other texturing.

Also, there is currently only unique artwork for four different directions: up, down, left, and right. G.R.E.T.A. is unique in that she takes longer to turn and can be facing a diagonal direction (i.e. up-left, up-right, down-left, and down-right) at any given time. We are currently rotating the existing pixel art by 45 degrees to achieve this, but this results in blurry artwork that is not pixel-perfect and is not from the correct perspective. Ideally we should have four new pixel art images for each of the diagonal directions with the correct perspective.

• [x] Add new pixel art for each diagonal direction
• [ ] Add more texture (e.g., dirt, scratches)

[albrow]

Diagonal directions were added in 89b52124c0b8dbe4316fe4d0b86008ef43c76e1a