Fix marker shapes with segments

[daschw]

Consider the following code:

using Plots

yv = ones(9)
ys = [1; 1; NaN; ones(6)]

plot(
   5 .- [yv 2ys 3yv 4ys],
   seriestype = [:path :path :scatter :scatter],
   markershape = [:utriangle, :rect],
   markersize = 8,
   color = [:red, :black],
)

Before

This is what we get on master with different backends. No backend behaves as expected for the top series.

GR

PyPlot

Plotly

PGFPlotsX

After

This PR fixes this for GR, PyPlot and Plotly. I did not manage to fix it for PGFPlotsX yet.

GR

PyPlot

Plotly

I added above example as a new test example. https://github.com/JuliaPlots/PlotReferenceImages.jl/pull/83is required for the tests to pass.

[daschw]

THis also fixes our broken doc image in http://docs.juliaplots.org/latest/input_data/#columns-are-series