Fixed drawing translucent 1px high polygons

[radarhere]

Resolves #6275. It doesn't change the behaviour of the code from that issue, but it addresses a concern raised there.

Background

PR #4333 adjusted polygon_generic to only draw each pixel once, so that repeated translucent pixels do not combine to produce a different final opacity.
PR #5835 limited this new approach to only run when there was translucency involved, for the sake of performance.

Issue #6275 has found a situation where #4333 changed not just the final opacity, but which pixels were drawn. Because #6275 doesn't involve translucency, #5835 changed it back, but the missing pixels are still missing when there is translucency involved.

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This PR fixes that unexpected change.

The scanline algorithm detects where edges intersect with each row of an image, storing them in xx.
It works its way along each row, keeping track with x_pos of the current position, so that pixels are not redrawn.
Horizontal lines do not contribute to the intersections, so it draws them separately as it goes, both partway through the row and at the end.
However, if there are no intersections (if j is zero), then when it goes to draw those end lines, they are skipped, as it thinks they are still to come.
So this PR instead sets x_pos to a special value if there are no intersections. If there are no intersections in this row, then the horizontal lines should just be drawn.