What should `default_brettel_neutral` be?

default_brettel_neutral is used as a reference value in the simulation of color deficiency.

Lines 63 to 65 in f38f27c

    
           # This is supposed to be "the brightest possible metamer of an equal-energy 
        
           # stimulus". I'm punting a bit and just calling that RGB white. 
        
           const default_brettel_neutral = convert(LMS, RGB(1, 1, 1))

Since sRGB has a white point of D65, this looks more straightforward with convert(LMS, WP_D65).
However, why is WP_D65 used instead of WP_E (equal-energy)?

If we change it, it may affect the default color scheme of Gadfly etc. Therefore, rather than change the behavior, I would like to make the third argument of protanopic etc. a public API and make the documentation explicit.

cc: @dcjones
GiovineItalia/Compose.jl@9c3a046

It probably should be WP_E, and I just didn't quite know what I was doing when I wrote that.

Changing it does affect Gadfly's default color scheme, but I don't find the change to be bad per se.

Here's what the change looks like, if anyone cares to disagree:

	# This is supposed to be "the brightest possible metamer of an equal-energy
	# stimulus". I'm punting a bit and just calling that RGB white.
	const default_brettel_neutral = convert(LMS, RGB(1, 1, 1))