Closes #8981

PR Summary

For images we do some pixel rounding, but if we are doing unsampled images (eg imshow(..., interpolation='none')) and saving to a backend that allows rasters (svg, pdf), then we don't want to do the pixel rounding - that just makes the image sizes fractionally too large when that isn't needed at all if we don't resample.

All the test images that changed just model my a pixel, and all have imshow(..., interpolation='none').

Old

New

code

import numpy as np
import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([0, 0, 1, 1, 0], [0, 1, 1, 0, 0], color='r', lw=0.1)
np.random.seed(19680801)
ax.imshow(np.random.randn(16, 16), cmap='Blues', extent=(0, 1, 0, 1),
            interpolation='none', vmin=-1,vmax=1)
ax.set_xlim(-0.1, 1+0.1)
ax.set_ylim(-0.1, 1+0.1)

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst