Last trailer comment has priority

[radarhere]

Suggestions for python-pillow#7382

• Group together some code to reduce duplication
• Break after reading "%%EOF"
• I found in https://web.mit.edu/PostScript/Adobe/Documents/5001.DSC_Spec.pdf under 4.4 that

In the case of multiple trailer comments (those comments that were deferred using the (atend) convention), the last comment encountered is considered to be the truth.

so I've altered the code to use the size from the last BoundingBox in the trailer.

[nopperl]

thanks for the improvements!