common fill: add fill-rule interface.

Fill rule is used to select how paths are filled.

For both fill rules, wheter or not a point is included in the fill is determined by taking a ray
from that point to infinity and looking at intersections with the path. The ray can be in any
direction, as long as it doens't pass through the end point of a segment or have a tricky
intersection such as intersecting tangent to the path.

@API Addtions:

enum class TVG_EXPORT FillRule { Winding = 0, EvenOdd };
Result Fill::rule(FillRule r) noexcept;
FillRule Fill::rule() const noexcept;

@Examples:

shape->rule(FillRule::EvenOdd);

@issue: 97

inc/thorvg.h

@@ -56,6 +56,7 @@ enum class TVG_EXPORT PathCommand { Close = 0, MoveTo, LineTo, CubicTo };
 enum class TVG_EXPORT StrokeCap { Square = 0, Round, Butt };
 enum class TVG_EXPORT StrokeJoin { Bevel = 0, Round, Miter };
 enum class TVG_EXPORT FillSpread { Pad = 0, Reflect, Repeat };
+enum class TVG_EXPORT FillRule { Winding = 0, EvenOdd };
 enum class TVG_EXPORT CompositeMethod { None = 0, ClipPath };
 enum class TVG_EXPORT CanvasEngine { Sw = (1 << 1), Gl = (1 << 2)};
 
@@ -239,12 +240,14 @@ public:
     //Fill
     Result fill(uint8_t r, uint8_t g, uint8_t b, uint8_t a) noexcept;
     Result fill(std::unique_ptr<Fill> f) noexcept;
+    Result fill(FillRule r) noexcept;
 
     //Getters
     uint32_t pathCommands(const PathCommand** cmds) const noexcept;
     uint32_t pathCoords(const Point** pts) const noexcept;
     Result fill(uint8_t* r, uint8_t* g, uint8_t* b, uint8_t* a) const noexcept;
     const Fill* fill() const noexcept;
+    FillRule fillRule() const noexcept;
 
     float strokeWidth() const noexcept;
     Result strokeColor(uint8_t* r, uint8_t* g, uint8_t* b, uint8_t* a) const noexcept;

src/lib/tvgFill.cpp

@@ -88,4 +88,4 @@ FillSpread Fill::spread() const noexcept
 Fill* Fill::duplicate() const noexcept
 {
     return pImpl->duplicate();
-}
+}

src/lib/tvgShape.cpp

@@ -384,3 +384,17 @@ StrokeJoin Shape::strokeJoin() const noexcept
 
     return pImpl->stroke->join;
 }
+
+
+Result Shape::fill(FillRule r) noexcept
+{
+    pImpl->rule = r;
+
+    return Result::Success;
+}
+
+
+FillRule Shape::fillRule() const noexcept
+{
+    return pImpl->rule;
+}

src/lib/tvgShapeImpl.h

@@ -196,6 +196,7 @@ struct Shape::Impl
     Fill *fill = nullptr;
     ShapeStroke *stroke = nullptr;
     uint8_t color[4] = {0, 0, 0, 0};    //r, g, b, a
+    FillRule rule = FillRule::Winding;
     void *edata = nullptr;              //engine data
     Shape *shape = nullptr;
     uint32_t flag = RenderUpdateFlag::None;
@@ -329,6 +330,7 @@ struct Shape::Impl
         if (!ret) return nullptr;
 
         auto dup = ret.get()->pImpl;
+        dup->rule = rule;
 
         //Color
         memcpy(dup->color, color, sizeof(color));
@@ -346,6 +348,7 @@ struct Shape::Impl
             dup->flag |= RenderUpdateFlag::Stroke;
         }
 
+        //Fill
         if (fill) {
             dup->fill = fill->duplicate();
             dup->flag |= RenderUpdateFlag::Gradient;