fix wrong bar width in ImageWriter

[fostermaier]

fixes #79 and maybe others?

This might seem trivial, but took me a couple of minutes to track it down. I'm attaching false-colored images for reference. Please pay attention to the final white bar in particular. SvgWriter is not affected as the rectangle implementation is different.

DPI 300

before

after

DPI 100

before

after

[fostermaier]

might also be related to #39

[codecov[bot]]

Codecov Report

Base: 80.58% // Head: 80.63% // Increases project coverage by +0.04% :tada:

Coverage data is based on head (6073331) compared to base (0489239). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #186 +/- ## ========================================== + Coverage 80.58% 80.63% +0.04% ========================================== Files 17 17 Lines 886 888 +2 ========================================== + Hits 714 716 +2 Misses 172 172 ``` | [Impacted Files](https://codecov.io/gh/WhyNotHugo/python-barcode/pull/186?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hugo+Osvaldo+Barrera) | Coverage Δ | | |---|---|---| | [barcode/writer.py](https://codecov.io/gh/WhyNotHugo/python-barcode/pull/186/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hugo+Osvaldo+Barrera#diff-YmFyY29kZS93cml0ZXIucHk=) | `95.08% <100.00%> (+0.04%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hugo+Osvaldo+Barrera). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hugo+Osvaldo+Barrera)

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[fostermaier]

Sorry about the noise. I just saw that I accidentally pushed to the PR branch. Feel free to cherry-pick.

The changes induced by the second commit, if merged, fix #30 as we are using it for borderless barcodes in our project.

[WhyNotHugo]

The changes in the resulting images look good to me. I haven't yet had a change to sit down and understand the code changes, I'll get back to you once I have.

[WhyNotHugo]

Thanks!