frontend: Make CropItem anchor logic stateless

[feeqra]

Uses an existing helper function to replace the manual stateful calculation of the stretch item's anchor point with a stateless calculation based on the final size of the stretch item. Reduces code complexity and redundancy. Improves readability.

Description

The original logic determines the stretch-item's new anchor point position using the mouse's state. This isn't necessary as this new position can be calculated from the stretch-item's final dimensions.

Motivation and Context

Removes redundant logic that was already handled by the helper function. Improves readability.

How Has This Been Tested?

OS: Ubuntu
Hardware: Local WSL build environment

Manual testing steps:

• Added an image to the canvas.
• Held alt and dragged all 4 edges and 4 corners independently.
• Verified that cropping and anchor point movement behaves as expected.

Types of changes

Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.