Make integer types used for sizes/width/height/stride/etc more consistent

In various places signed integers are used because that's what is used in C, but the values can never be negative (-> assertion needed, but also for the unsigned case!). Similarly, sometimes u32 and sometimes usize is used.

This should be made more consistent.

The main question is what type to use. For stride at least, usize makes most sense because it's used for memory accesses. For others, usize might make code more convenient because fewer casts are needed, but maybe not.