Remove the `n_quadrants` field because it is entirely redundant to the size of `shield_quadrant`. Also, be more rigorous about the difference between `shield_get_quad(i)` (which can fluctuate) and `shield_quadrant[i]` (which gives the exact quadrant strength). Finally add some C++ casts and move some checks to better places.
This refactors the object_h struct to store objects as indexes rather than pointers. This is a prerequisite for making the collection of objects dynamic in the future. See also #6100.
Follow-up to #5800 and #5829.
This maintains the type information which can be used for enforce
const-correctness and also for implementing more advanced ways of
handling the type information.
Goober5000