SDF bug fixes

I found another stride bug and fixed some docs.

Please check if my fix for issue https://github.com/memononen/SDF/issues/2 makes sense.

I created a font stash fork too and cleaned up my hacks a bit. I'll commit my stuff shortly. It's still not the nicest way to integrate SDF to font stash but at least it's something to play with.
I think we should make this a bit more obvious and have "inline" comment too. i.e.
```C
// Skip flat areas.
if (img[k] == 0) continue;
if (img[k] == 255) {
	// Special handling for cases where full opaque pixels are next to full transparent pixels.
	// See: https://github.com/memononen/SDF/issues/2
	if ((img[k-1] != 0) && (img[k+1] != 0) && (img[k-stride] != 0) && (img[k+stride] != 0)) continue;
}
...
```