Feat: PlotPolygon

[brenocq]

Closes #631

This PR introduces PlotPolygon as requested by #631.

// Flags for PlotPolygon. Used by setting ImPlotSpec::Flags.
enum ImPlotPolygonFlags_ {
  ImPlotPolygonFlags_None     = 0,       // default (closed, convex polygon)
  ImPlotPolygonFlags_Concave  = 1 << 10, // use concave polygon filling (slower but supports concave shapes)
};

// Plots a polygon. Points are specified as separate x and y arrays. Supports both convex and concave polygons.
IMPLOT_TMP void PlotPolygon(const char* label_id, const T* xs, const T* ys, int count, const ImPlotSpec& spec=ImPlotSpec());

Usage

...
if (ImPlot::BeginPlot("Polygon Plot", ImVec2(-1,0), ImPlotFlags_Equal)) {
    ImPlot::PlotPolygon("Triangle", tri_xs, tri_ys, 3, {
        ImPlotProp_FillAlpha, 0.5f,
    });
    ImPlot::PlotPolygon("Pentagon", pent_xs, pent_ys, 5, {
        ImPlotProp_FillAlpha, 0.5f,
        ImPlotProp_FillColor, ImVec4(0,1,0,1),
    });
    ImPlot::PlotPolygon("Star (Concave)", star_xs, star_ys, 10, {
        ImPlotProp_FillAlpha, 0.5f,
        ImPlotProp_FillColor, ImVec4(1,1,0,1),
        ImPlotProp_Flags, ImPlotPolygonFlags_Concave,
    });

    ImPlot::EndPlot();
}

[brenocq]

Since the user specifies the points in plot space, but the ImGui draw list methods expect the points in screen space, I needed dynamically allocate an array of ImVec2 to convert each point from plot space to screen space before sending the array to ImGui, I think this is fine for now for an initial implementation, in the future we can create a polyline renderer so we avoid this dynamic allocation by filling in the draw list directly.