Add const specifiers, small efficiency boosts.

Add:
	src/matrix.(c, h)
		-virtually all matrix functions accept a Matrix_t pointer; to allow some
		compiler optimizations, add the const specifier whenever a Matrix_t
		pointer, or the Matrix_t pointed, would not be modified.

	src/utils.c
		-add optimizations to drawLine():
			-replace each of the three if-else conditionals that set the values
			of dx1, dy1, dx2, and dy2 with a simple if-else, because the second
			conditional was, practically speaking, unnecessary.

			-replace the math-library abs() call with with an ABS() macro,
			because it's succinct and obviates adding abs() to the stack.