Gfx::drawLine still has a minor pixel-perfection issue

[AaronVanGeffen]

Version information:

• Platform: any
• Version: 24.09.1
• Build/commit: 9089129d709fa9729cacddf276aa2a8bde4fe6c6

Even after #2657, there still appears to be an issue with horizontal-to-vertical transitions in drawLine calls relative to vanilla. Per https://github.com/OpenLoco/OpenLoco/pull/2646#pullrequestreview-2372426739:

I checked the pixels and its so close, I notice in your version that some parts of the line are thicker (these are the 'lighter' pixels here, seen under a subtract filter in photoshop)

I ran it a second time just to be sure, and the exact same pixels are different, so its deterministic. Judging by the error, I'd say its an off-by-one somewhere?