Please describe the problem:
When using the trim tool (the one triggered by Shift+C), setting a fixed proportion, the proportion is not always kept.

Steps to reproduce:
1. Open a photo (say 2048x1536)
2. Use the trim tool (Shift+C)
3. Set a fixed proportion of 1:1 (square)
4. Start at the center of the photo (1024x768)
5. Draw a selection with the trim tool and try to go beyond the photo limit


Actual results:
Though the fixed proportion are set to 1:1, you will be able to do a trim of the lower bottom part of the photo, a region of 1024x768 pixels, not 1:1 proportion.

Expected results:
You shouldn't be able to select nothing more than 768x768 pixels.

Does this happen every time?
Yes

Other information:
This worked well in the 2.3.16 release.