I finally figured out what was up with my scaling.

It was all down to the fact that Spritesheets get automatically given an 8-pixel gap around all sprites. The maths that resulted from that slight tweak were insane!!!

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So, now that that's fixed, I've gone on to add basic primitive ovals, as well as Quads.
I'm now having to rethink how to do Lines, since apparently the OpenGL Line Width command isn't universal. I'm instead going to have to work out angles and render a quad over the area that line would be, given the current thickness.
Bit longwinded, if I'm honest, and really these primitive shapes/objects really aren't all that efficient!
I expected "Draw giant image on the screen" to be much more intensive than "Try to figure out the shape of a rectangle if the rectangle's supposed to be a thick and chunky line on the screen"

*tsk*

Anyhoo, most of the primitives are now in place, and I'll probably never use them. Ho hum..

I'll figure out the lines, tomorrow. Time for a rest, methinks!