Added duDumpHeightfield

A better format would be to first count the number of spans, write span count, and then store each individual span. No need to store x,y.
```C
int spanCount = 0;
for (const rcSpan* s = hf.spans[x + y * hf.width]; s; s = s->next)
	spanCount++;
io->write(&spanCount, sizeof(spanCount));
for (const rcSpan* s = hf.spans[x + y * hf.width]; s; s = s->next)
{
	unsigned short smin = s->smin;
	unsigned short smax = s->smax;
	unsigned char area = s->area;
	io->write(&smin, sizeof(smin));
	io->write(&smax, sizeof(smax));
	io->write(&area, sizeof(area));
}
```