More snap-points debug display...

Thought I would get some crude snapping to my snap-points in today, but the first thing I realised is that although I know where they are, I don't know what way up they are facing, so I quickly swapped my Vector2 SnapPoints for a struct containing a position and an orientation...

I then realised that to display the points, I really needed them sometimes to extend beyond the sheet edge.  However I want the sheet to clip to its edge so that it is easy, later, to put an image placed so it crosses the edge, and not see the bit outside the sheet.  Think of a coffee ring that extends off the edge of this piece of paper, for example...

So I reworked the clip-point debug display into a separate DebugOverlay object.

Then I found left-half snap-points we rotated correctly, but right-half not...  cue lots of hunting through code, examining transforms, checking how I create the data, etc etc...  only to eventually realised I had copy/pasted the snap-point draw loop *before* it had any rotation awareness, and only fixed that in the first copy <face palm>...

After which I converted the debug symbol for them into green half arrows for left, and ref half arrow for right, so that when they are docked together property they look like this:

The arrow is pointing at the actual snap-point, FWIW...

And then it was now and I thought I would write it up, and call it a day.