The cube example shouldn't use integers for texture coordinates

[ucarion]

This isn't a bug, but rather a suggestion for making the cube example more helpful for noobs (like me).

In the cube example, a_tex_coord is an ivec2/[i8; 2], which led me to assume that it is supposed to be an index. Of course, if I knew how texture() works in fragment shaders, I would have known better -- but I didn't, because I'm a noob. I assumed that a_tex_coord was a 2d-index into my texture, rather than a point in the unit square.

Perhaps a_tex_coord should be changed to be of the type vec2 in GLSL and [f32; 2] in Rust? Having it be an integer is useful only in the case of a texture of size no greater than 2x2, which is a rare use-case.

If there's no reason against making this change, I'll gladly open a PR myself. But I have the suspicion that I simply don't have the relevant context, and that there's a good reason the code is as-is.