city example gameplay is not fluid when optimised by the compiler

Hello,

I've been trying the “city” demo, bundled with small3dlib, on my laptop, and I've noticed that, when compiling the game with any optimisation flag greater than -O0, the car doesn't move forward fluidly, and can even be stuck in a sideway position.

I'm unable to pinpoint which optimization is affecting which line of code for now.

I'm compiling and running this example under Arch Linux, with GCC 12.2.0, on an Intel i7-1165G7.