softgpu: Fix double rectangle drawing at halfpixel

[unknownbrackets]

There seems to be some odd behavior with the X start pos, but at least the end pos should not be drawn twice when using 0.5 - 0.5.

This unfortunately only fixes some of the cases from #16131, as there are more problems after all. It helps:

• Frogger, which no longer draws a pixel twice.
• Dig Dug, same.
• MTX Mototrax, but only one of the lines, so there's more to it.
• Phantasy Star Portable 1, from not overdrawing.

I also checked several of the usual suspects (Valkyrie Profile, bloom in Call of Duty, etc.) which render correctly.

Triangles need some tweaks too, but there's some complexities there, and this fixes bugs already by itself.

-[Unknown]