Fixed coordinate systems

Fixed the handling of coordinate systems, especially proper Y-flip-or-not-to-flipping.

• The default coordinates are now device points with Y0 starting at the bottom (Y+ visually upward, matching the GLK default), the same but with Y0 at the top for the overlay layer (Y+ visually downward, matching UIKit/Quartz), and Y+ upward with the origin shifting for the content layer.
	• CGAffineTransform and CTM manipulations added to SceneForCG in drawOverlayLayerWithin:forView: and view:drawInRect: to orient things properly (Y-flipping).
	• GLKMatrix and modelview manipulations added to SceneForGLK in drawOverlayLayerWithin:forView: and drawContentLayerWithin:forView: to orient things properly (Y-flipping).
		• Content layer viewFocus Y-flip hack removed as a result.
• Scene#shiftFocusBy: replaced by shiftFocusByScreenPx:, shiftFocusByScenePx:, and shiftFocusBySceneIn:, to differentiate shifting by screen coordinates, GL scene coordinates, and GL scene coordinates within the pixels-to-inches transformation.
	• Also, viewFocus_px has been renamed to viewFocus_pt to match Apple's usage of the terms pixels (literal pixels) vs. points (retina-disregarded).  Related uses of `_px` updated to `_pt` as well.
• Scene now determines the PPI, and everything under the Scene (VisReps) use that.
	• Scene's screenPPIRetinaDisregarded and screenPPI accessors replaced with a definite PPI.
	• VisReps are now initialized with a `Scene *` rather than a PPI value, and use that weak reference to it to get the PPI when needed.  A bunch of the PPI setting and validating junk removed from the VisRep implementations as a result.
• Replaced BackgroundVisRep's inverse-current-CTM hack with shift based on the Scene's current viewFocus.
	• However, an issue remains that the BackgroundVisRep's pattern phase offset needs to be y-flipped (again, after all the other transformations).  I don't know why; perhaps pattern drawing ignores the CTM.

Minor:
• Some clarity comments here and there, especially regarding orientation.
• Tested an @todo regarding text overdraw; there are no issues, comment removed.
• Moved the screen-coord blue circle to 3/4 of the way down the screen— it's going away soon anyway.
• A couple of uses of drawRect replaced with viewSize (view.bounds.size).  I really need to make a full-pass and fix all of these— drawRect is for culling, not positioning.
• Some direct accesses of the _viewFocus_pt ivar replaced with proper getter calls.