GPencil: Improve Vertex Paint in filled areas

When use the Tint tool, it was very difficult to tint the filled areas if the strokes had very few points or the area was big.

Differential Revision: https://developer.blender.org/D7936

source/blender/editors/include/ED_gpencil.h

@@ -317,6 +317,10 @@ bool ED_gpencil_stroke_check_collision(struct GP_SpaceConversion *gsc,
                                        float mouse[2],
                                        const int radius,
                                        const float diff_mat[4][4]);
+bool ED_gpencil_stroke_point_is_inside(struct bGPDstroke *gps,
+                                       struct GP_SpaceConversion *gsc,
+                                       int mouse[2],
+                                       const float diff_mat[4][4]);
 
 #ifdef __cplusplus
 }