Initial `surfaceSTToTexelUV()` & GLKVector2 math

• New `surfaceSTToTexelUV()` function is a naïve first-pass at mapping the surface’s point-bounded coords to texel coords (and thus, pixels).
	• Switched `genDestImagePixelBytesAtPosition()` to generate texel UVs via `surfaceSTToTexelUV()` and use those values for the R and G channel values.
• Switched `genDestImagePixelBytesAtPosition()` and `surfaceSTToTexelUV()` to use GLKVector2 (instead of CGPoint) and related NEON-optimized GLK math functions.
	• Added GLKit framework to the project.
	• Created two new specialized functions within ViewController— `GLKVector2LengthSqr()` and `GLKVector2DistanceSqr()`.  Both have the same logic as their non-Sqr-suffixed couterparts, except that they don’t perform the final `sqrt()` operation.
		» It’s a little surprising that Apple left these out— squared calculation are faster and just as useful for some needs and the post-sqrt values.
		» Using these in `surfaceSTToTexelUV()`.