Overloaded the ^ binary operator for struct Pixel (it now performs color inversion using the second pixel as a switch
Overloaded unary negation operator for struct Pixel
Overloaded stream output << operator for struct Pixel
Moved the write_test_image() function to test_images.h
Changed struct Pixel to struct Color, since that's what it represents
Renamed struct Point to struct Pixel, since it is limited to describing pixel locations only