Before this patch, in the event of `q->image` reallocation success but
`q->pixels` failure, `q->image` would point to a freed memory.
After this patch, once quirc_resize() returns, `q->image` consistently
point to a memory address that can be freed.
There is still an inconsistency left in the example codepath: `q->{h,w}`
would hold the "old" values, while `q->image` allocated size would be
based in the requested width/height; hence the comment update about the
state of the QR-code recognizer (it should only be passed to
quirc_destroy()).
Fix#2, this patch makes large images support a compile time option by
allowing QUIRC_MAX_REGIONS to be tuned via CFLAGS up to 65534 (current
default is 254).
