Crop: compositional guides

[dsizzle]

Crop would be more useful with an option to show some basic compositional guides by drawing grid lines on the cropped view.

For example:

• rule of thirds (split into thirds vertically and horizontally)
• centerlines (split in half vertically and horizontally)
• X (top-left to bottom-right and vice versa)
• "phi grid"/golden mean grid (split into thirds, but with the ratio 1:0618:1)
• golden ratio? might be hard, not sure how to draw the spiral.