Make minimum line thickness dependent on AA level.

The PDF spec says that line thickness of 0 should mean "1 device
pixel". We have been doing some dodgy logic where if the line
thickness as scaled by the ctm is small (< 0.1f), make it at
least 1 device pixel.

This can mean that a line can not qualify for being thickened at
36dpi, but can be thickened at 24dpi. The thickened line at 24dpi
is much thicker than the unthickened line at 36dpi, meaning that
we get a noticable shift in rendering.

Why do we do this strange logic? Well, presumably it's to avoid
thin lines dropping out completely.

We therefore move to some new logic. Firstly, we create a fudged
'aa_level' value, dependent on the antialias level. With AA level
0 (no antialiasing), this corresponds to 1 device pixel. For
maximum AA level (8), this corresponds to 1/5 of a device pixel.

Thus we should get 'continuous' results across different dpis.

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