0027104: DownCast() cannot return null for mismatched handle

0027104: DownCast() cannot return null for mismatched handle

Method DownCast() is made template, to be available only when argument is actually a pointer or handle to a base class.

For compatibility with existing code, method DownCast() that can be used for the same type, derived, or unrelated class (i.e. where there is no actual down casting) is still available, its use shall cause "deprecated" compiler warning.

OCCT code is updated to remove meaningless DownCast()s; a few places where DownCast() was used with argument of unrelated type are corrected.

DRAW command QAHandleCast is removed (it was useful only during redesign of handles).