Resolve some of the -Wsign-compare warnings

[vpa1977]

During compilation libde265 produces a number of -Wsign-compare warnings:

~/git/libde265/libde265/decctx.h: In member function ‘slice_unit* image_unit::get_next_slice_segment(slice_unit*) const’:
~/git/libde265/libde265/decctx.h:226:20: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<slice_unit*>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
  226 |     for (int i=0; i<slice_units.size()-1; i++) {
      |                   ~^~~~~~~~~~~~~~~~~~~~~

this PR resolves some of them.

It replaces int with size_t for index variables and does not have a functional impact.

[farindk]

Thank you for the hard work.

I have to say that I'm no fan of size_t and consider this a design mistake of the std-library. It induces too many potential bugs (like in for-loops counting down) and that unsigned type spreads like cancer. I'd like to replace all of that with C++20 ssize() at a later point in time.

But all your changes are correct and non-problematic. Thus, I'm going to merge this.