AREA_TRIANGLE
#define AREA_TRIANGLE(x0,y0, x1,y1, x2,y2) \
    (x0*y1+x1*y2+x2*y0 - x0*y2-x2*y1-x1*y0)
   求平面上的三角形的面积。结果的正负与三个顶点的顺序有关。各种 TrianglePainter 依赖它。

相关:sjz::TrianglePainter
   sjz::TrianglePainter_P2FC3F
   sjz::TrianglePainter_P4FC3F
   sjz::TrianglePainter_P4FT2F