Feat: ImPlot3DSpec

[brenocq]

Closes #166

As ImPlot is moving towards ImPlotSpec API as a replacement for the SetNextXX API (epezent/implot#519), ImPlot3D should do the same.

All plotting functions now take ImPlot3DSpec as argument.

How to use ImPlot3DSpec

There are two ways of populating the ImPlot3DSpec:
1. By declaring and defining a struct instance:

ImPlot3DSpec spec;
spec.LineColor = ImVec4(1,0,0,1);
spec.LineWeight = 2.0f;
spec.Marker = ImPlot3DMarker_Circle;
spec.Flags = ImPlot3DItemFlags_NoLegend | ImPlot3DLineFlags_Segments;
ImPlot3D::PlotLine("Line", xs, ys, zs, 100, spec);

2. Inline using ImPlot3DProp,value pairs (order does NOT matter):

 ImPlot3D::PlotLine("Line", xs, ys, zs, 100, {
 ImPlot3DProp_LineColor, ImVec4(1,0,0,1),
 ImPlot3DProp_LineWeight, 2.0f,
 ImPlot3DProp_Marker, ImPlot3DMarker_Circle,
 ImPlot3DProp_Flags, ImPlot3DItemFlags_NoLegend | ImPlot3DLineFlags_Segments
});

ImPlot3DSpec enum and structs:

// Plotting properties. These provide syntactic sugar for creating ImPlot3DSpec from (ImPlot3DProp,value) pairs
enum ImPlot3DProp_ {
    ImPlot3DProp_LineColor,       // Line color; IMPLOT3D_AUTO_COL will use next Colormap color
    ImPlot3DProp_LineWeight,      // Line weight in pixels
    ImPlot3DProp_FillColor,       // Fill color (applies to shaded regions); IMPLOT3D_AUTO_COL will use next Colormap color
    ImPlot3DProp_FillAlpha,       // Alpha multiplier (applies to FillColor and MarkerFillColor)
    ImPlot3DProp_Marker,          // Marker type
    ImPlot3DProp_MarkerSize,      // Size of markers (radius) *in pixels*
    ImPlot3DProp_MarkerLineColor, // Marker outline color; IMPLOT3D_AUTO_COL will use next LineColor
    ImPlot3DProp_MarkerFillColor, // Marker fill color; IMPLOT3D_AUTO_COL will use LineColor
    ImPlot3DProp_Offset,          // Data index offset
    ImPlot3DProp_Stride,          // Data stride in bytes; IMPLOT3D_AUTO will result in sizeof(T) where T is the type passed to PlotX
    ImPlot3DProp_Flags            // Optional item flags; can be composed from common ImPlot3DItemFlags and/or specialized ImPlot3DXFlags
};

...

struct ImPlot3DSpec {
    ImVec4 LineColor = IMPLOT3D_AUTO_COL;        // Line color; IMPLOT3D_AUTO_COL will use next Colormap color
    float LineWeight = 1.0f;                     // Line weight in pixels
    ImVec4 FillColor = IMPLOT3D_AUTO_COL;        // Fill color (applies to shaded regions); IMPLOT3D_AUTO_COL will use next Colormap color
    float FillAlpha = IMPLOT3D_AUTO;             // Alpha multiplier (applies to FillColor and MarkerFillColor)
    ImPlot3DMarker Marker = ImPlot3DMarker_Auto; // Marker type
    float MarkerSize = IMPLOT3D_AUTO;            // Size of markers (radius) *in pixels*
    ImVec4 MarkerLineColor = IMPLOT3D_AUTO_COL;  // Marker outline color; IMPLOT3D_AUTO_COL will use LineColor
    ImVec4 MarkerFillColor = IMPLOT3D_AUTO_COL;  // Marker fill color; IMPLOT3D_AUTO_COL will use LineColor
    int Offset = 0;                              // Data index offset
    int Stride = IMPLOT3D_AUTO;                  // Data stride in bytes; IMPLOT3D_AUTO will result in sizeof(T) where T is the type passed to PlotX
    ImPlot3DItemFlags Flags =
        ImPlot3DItemFlags_None; // Optional item flags; can be composed from common ImPlot3DItemFlags and/or specialized ImPlot3DXFlags

⚠️ Breaking changes

• SetNextLineStyle has been removed, styling should be set via ImPlot3DSpec.

  // Before
  ImPlot3D::SetNextLineStyle(line_color, line_weight);
  ImPlot3D::PlotLine("Line", xs, ys, zs, count);
  
  // After
  ImPlot3DSpec spec;
  spec.LineColor = line_color;
  spec.LineWeight = line_weight;
  ImPlot3D::PlotLine("Line", xs, ys, zs, count, spec);

• SetNextFillStyle has been removed, styling should be set via ImPlot3DSpec.

  // Before
  ImPlot3D::SetNextFillStyle(fill_color, fill_alpha);
  ImPlot3D::PlotTriangle("Triangle", xs, ys, zs, count);
  
  // After
  ImPlot3DSpec spec;
  spec.FillColor = fill_color;
  spec.FillAlpha = fill_alpha;
  ImPlot3D::PlotTriangle("Triangle", xs, ys, zs, count, spec);

• SetNextMarkerStyle has been removed, styling should be set via ImPlot3DSpec.

  // Before
  ImPlot3D::SetNextMarkerStyle(marker, marker_size, fill_color, line_weight, marker_outline_color);
  ImPlot3D::PlotScatter("Scatter", xs, ys, zs, count);
  
  // After
  ImPlot3DSpec spec;
  spec.LineWeight = line_weight;
  spec.Marker = marker;
  spec.MarkerSize = marker_size;
  spec.MarkerLineColor = marker_outline_color;
  spec.MarkerFillColor = fill_color;
  ImPlot3D::PlotScatter("Scatter", xs, ys, zs, count, spec);

• Flags, Offset and Stride should also be set via ImPlot3DSpec now.