UIPageViewController with swipe-change between CG and GL

Added a root UIPageViewController that makes it possible to swipe the view to switch between CoreGraphic rendering (SceneControllerForCG) and OpenGL rendering (SceneControllerForGLK) at runtime.

As a result:
• AppDelegate's viewController property replaced with rootViewController (pointing to the UIPageViewController) and currentSceneViewController properties.
	• Likewise, scene replaced with currentScene, which is now just a pass-through to the currrentSceneViewController's scene (AppDelegate no longer claims ownership of the Scene).
	• currentSceneViewController does a fairly safe lookup that returns nil unless UIPageViewController gives us at least one currently visible page/viewController and is indeed a SceneViewControllerProtocol adhering one.
• AppDelegate now creates and owns SceneViewControllerForCG and SceneViewControllerForGLK instances.
• Added delegate and dataSource methods to AppDelegate to feed the UIPageViewController as needed, and to handle switching between SceneViewControllers.
	• Currently, the Scenes' states are synced on switch.  Eventually, this should be shared data.
• Overlay line added to both CG and GLK Scenes displaying the current renderer (CoreGraphics or OpenGL).
• Framerate display changed to display all three metrics: preferred, reported (just the `framesPerSecond` property), and measured.
	» Though measured isn't implemented yet for SceneControllerForGLK.

Minor:
• The GLKViewController delegation `update` method added to SceneViewControllerForGLK.  We'll probably need this soon.
• kFramesPerSecond constant in AppDelegate.
• AppDelegate initSceneViewControllers and initRootViewController methods broken out.
• AppDelegate#dealloc removed in favor of doing all deconstruction in applicationWillTerminate:, which is never* called.
• A few `#pragma mark` sections added here and there.
• Comments added here and there.

* Note function-doc comment added explaining this.
SceneView display (right here, right now)

Added a display method to SceneView, which talks to the CALayer to get it to call drawLayer:inContext (also added) immediately, which calls drawRect: immediately (rather than waiting until the next update).  Changed SceneViewControllerForCG over to use this.

Also, draw at the beginning of the PageViewController transition preempted so that the views match when the page starts peeling.  This is currently only needed for CoreGraphics (since the GLKViewController seems to annoyingly unpause itself automatically).
Improvements to SceneForGLK's texture alignment

It turns out that the fuzziness seen when switching over to linear interpolation (GL_LINEAR for GL_TEXTURE_MIN/MAG_FILTER) is caused by the 4X multisampling, not misalignment.  Now the poly has proper alignment (no inset/outset necessary) that looks right with nearest interpolation (GL_NEAREST) or linear with no multisampling.

Other stuff that happened:
• Removed TexturedVertex2DCoordsMake; it wasn't really that helpful.
• Added kGraySaturation, just to make things a bit easier to read.
• Release (= nil) lines for each of the VertexBuffers and TextureInfos in unload.  Missed this, not that I didn't put the textures together in a rush.
• Renamed and modified the var grouping a bit in drawOverlayText: so that they're a bit easier to understand.
• Added error handling for GLKTextureLoad::textureWithGCImage: call.
• Removed GL_TEXTURE_WRAP_S/_T clamp (GL_CLAMP_TO_EDGE).  I don't think it's really necessary, and it doesn't get set back anywhere else.
A couple minor utilities and changes

• New CGAdditions.h/m files with NSStringFromCGImageAlphaInfo() and NSStringFromCGBitmapInfo().  They do what you'd expect.
• “computed screen PPI (lookup-based guess)” was somewhat contradictory.  Now it's just “screen PPI (lookup-based guess)”
• Added a DO_PRAGMA macro I found in the GCC docs to the prefix file.  May need it later.
Removed UIPageViewControllerDelegate debug logging code from AppDelegate

(Moving it to debug/refactor/re-gl-ize branch.)