vkgears: fix non-gnu compilation
We can't use powf() as a compile-time function, which is required for a global const array initializer.
So let's move this into the draw_gears() function instead. This is what we do for the other arrays here as well, and a sufficiently clever compiler should be able to tell that it's not being modified anyway.
Fixes: a92d6f62 ("vulkan: add vkgears demo") Closes: #30 (closed)