Undefined behavior in the implementation of `nk_ptr_add`

On adding an integer to a pointer, the C standard says

If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

This means this usage of nk_ptr_add triggers undefined behavior when b->size < size because it causes an overflow, and this can happen when the buffer needs to grow. In fact, I caught this error when trying to compile and run Nuklear for a Zig project, since Zig catches various kinds of undefined behavior in debug mode.

I think the solution is to cast the i parameter of nk_ptr_add to a signed type like ptrdiff_t to avoid the overflow. This requires adding a new type to the list of integer types here.

Start in nuklear.h at nk_ptr_add and inspect the integer type definitions around lines 418-427, then follow its use near line 8630. Verify the pointer arithmetic when the buffer grows and b-size is less than size; done means the operation no longer invokes signed or unsigned overflow under the stated case.