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Texture gen X/Y›Red/Green coloring & cleanup

• Changed `genDestImagePixelBytesAtPosition()`’s (formerly `genDestImageBytesAtPosition()`) pixel color logic to color red from calculated `pixelU` value, green from calculated `pixelV` value, and alpha to fully opaque.
• New `genDestImageBytesAtPosition()` function (replacing the previous same-named function) interprets the byte request by chopping it into pixel-aligned 4-byte chunks, then asking `genDestImagePixelBytesAtPosition()` for each pixel of data.
• `destImage` getter now caches the destImage’s width/height and totalBytes as ivars for quick lookup in the gen functions.
• Minor: Removed `_imageView.image` set in viewDidLoad; `pointsDidChange` takes care of this and is already called here.
	• Fix: In viewDidLoad, setting the `_srcImage` earlier so that its ready when `pointsDidChange` is called.