remove black lines from GR surface with equal-length x, y, and z input

[daschw]

changes

using Plots

x_range = -10:0.5:10
p_range = -3:0.5:3

f(x, p) = sin(x) + cos(p)

xs = Float64[]
ps = Float64[]
ws = Float64[]

for x in x_range, p in p_range
    push!(xs, x)
    push!(ps, p)
    push!(ws, f(x, p))
end

surface(xs, ps, ws)

from

to

fix #3279 cc @foldfelis