Read bounding box information from the trailer of EPS files if specified

[nopperl]

The bounding box for EPS images is currently only parsed from the file header. However, the DSC specification also allows it to be specified in the trailer (see Section 4.6 of the specification). According to the specification, this must be indicated in the header using %%BoundingBox: (atend). The real bounding box information is then specified at the end of the file following the %%Trailer comment. For an example, see Tests/images/zero_bb_trailer.eps.

Currently, the EpsImagePlugin tries to parse the bounding box only from the header, which fails if the value is (atend). This PR fixes the error and implements boundary box parsing from the trailer.

[radarhere]

I'm guessing you created the test image by opening up one of our existing test images in a text editor and modifying it manually?

[nopperl]

Yes, I simply modified zero_bb.eps because I can't distribute the images I've worked with. Should it be a "real" image instead?

[radarhere]

Not necessarily, no. Real images are certainly ideal, but created ones are also accepted, as it's not always possible/practical to find an image that triggers a certain scenario.