Fix preconditions and generic types

[yeldarby]

The preconditions prevented reading row, column, and channel 0. And the lack of a type constraint on the generic classes were preventing from creating an extension that performed arithmetic operations on the pixel data.

(eg this would fail because yo ucouldn't compare image[r, c, 0] to 0; constraining <T> to a BinaryInteger should fix it (and it wouldn't even make sense for <T> to be something weird like a UIView or something like that):

extension SGLImage {
	func toASCII() {
		let image = self

		var output = ""
		for r in 0..<image.height {
			if(r % 10 != 0) { continue }
			for c in 0..<image.width {
				if(c % 10 != 0) { continue }
				
				if(image[r, c, 0] > 0) {
					output += "+"
				} else {
					output += " "
				}
			}
			output += "\n"
		}
	}
}

(Fixes SwiftGL#2)