Store TIFF DPI as integer

[masterfool]

Fixes #2077 which prevents TIFF from being saved as JPEG with its original DPI.

Stores the TIFF DPI as integers

[arjennienhuis]

I think it makes more sense to round when saving the image. Now you always lose precision, even when not converting the image to JPEG.

[masterfool]

Good idea. I've updated the pull request.

[wiredfool]

-1

Tiff resolution is stored as a rational value and a resolution unit. See page 18/19 here: https://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf

If there's an issue with jpegs, then either:

1. The jpeg spec doesn't allow rational dpi, and we're enforcing that in a confusing way
   or
2. We're not properly allowing a rational dpi in the jpeg save routine.

Either way, we need to dig a bit in the jpeg spec and fix the jpeg side.

[arjennienhuis]

https://www.w3.org/Graphics/JPEG/jfif3.pdf

page 5

integer dpi or dots per cm

[wiredfool]

That certainly looks like #1 is the issue, and a better way to enforce integer dpi in jpegs would do it.

[arjennienhuis]

see #2084

[wiredfool]

Closing in (eventual) favor of #2084