Pixels can be annotated with symbols.  Can be activated or
 deactivated by checkbox.
Made size and spacing of pixels into global variables (pixel_width
 and pixel_step, respectively) to make it easier to change
 values when necessary.  May allow user to change as interface.
In implementing symbols, introduced a function for calculating
 an RGB colour that is substantially different from a given colour
 (invertColour).
Change dimensions of mouse-over rounded box depending on colour
 mode (i.e., RGB versus DMC, etc.)