Perspective texture warping

I did a texture test for part of the play area. The results were not what I'd hoped for:

All of these lines should be straight! After triple checking my code to make sure I wasn't calling any functions like 'drawWonkyLine' and 'activateDrunkGraphicsMode', and ruling out floating point inaccuracies, I sought the help of the developers of the graphics library I'm using. After some discussion and experiments, it's been agreed that there's a bug in the way textures are drawn with perspective, and I have been provided with a temporary workaround, which results in this:

Much more promising! Unfortunately the workaround is expected to have some other side effects and also prevents me from making use of orthographic projection, which I planned to use for GUI elements - but I can work around that for now. Big thanks to kazade for his support with this.