src/output/driver.c: Change engine_stack from an array to a list.

src/output/driver.c:  Change engine_stack from an array to a list.

The structure called engine_stack was formerly implemented as a heap
based array allocated with realloc.   However when realloc moves data
it makes only shallow copies and does not copy referand of pointers.
This was causing crashes under certain circumstances.

Rather than trying to fix this, this change re-implements engine_stack
using the linked list structure from ll.h