Returns True after converting the image to monochrome.
All plug-in functions must return True or False. This function returns True
because it modifies the image. It converts the image to either greyscale or
sepia tone, depending on the parameter sepia.
If sepia is False, then this function uses greyscale. For each pixel, it computes
the overall brightness, defined as
0.3 * red + 0.6 * green + 0.1 * blue.
It then sets all three color components of the pixel to that value. The alpha value
should remain untouched.
If sepia is True, it makes the same computations as before but sets green to
0.6 * brightness and blue to 0.4 * brightness.
Parameter image: The image buffer
Precondition: image is a 2d table of RGB objects
Parameter sepia: Whether to use sepia tone instead of greyscale
Precondition: sepia is a bool