Fix for issue [Cairo]: Arrows have a "gap" (#561)

The problem to fix is that the arrow drawn by draw_arrow1() in src/fl_symbols.cxx
displays a faint clear line between the stem and head of the arrow with the Cairo
graphics driver.

This occurs because draw_arrow1() draws the arrow in 2 steps (a rectangle +
a triangle) and the Cairo driver is configured to use antialiasing when filling
polygons. The antialiasing produces the faint line between stem and head.

Why does draw_arrow1() draw a rectangle + a triangle rather than a
7-vertex polygon? That's because the X11 graphics driver fails with its polygon-
drawing function when the polygon is also rotated: the polygon is drawn
empty.

We want to keep using antialiasing under Cairo for polygons because
the result is better with non horizontal/vertical polygon edges.

This implementation changes function draw_arrow1() which draws
the arrow as a 7-vertex filled polygon except when the graphics driver
returns false for its virtual member function can_fill_non_convex_polygon().
In that situation, draw_arrow1() draws, as before, a rectangle + a triangle.
The new, virtual member function can_fill_non_convex_polygon() returns
true except for the X11 graphics driver. Therefore, draw_arrow1() is effectively
unchanged under the X11 driver.

1 files changed, 2 insertions, 0 deletions

diff --git a/FL/Fl_Graphics_Driver.H b/FL/Fl_Graphics_Driver.H
index 0f25756d6..e131a2961 100644
--- a/FL/Fl_Graphics_Driver.H
+++ b/FL/Fl_Graphics_Driver.H
@@ -316,6 +316,8 @@ public:
   virtual void fixloop();
   virtual void end_polygon();
   virtual void end_complex_polygon();
+  // default implementation is most probably enough
+  virtual bool can_fill_non_convex_polygon() { return true; }
   virtual void gap();
   virtual void circle(double x, double y, double r);
   virtual void arc(double x, double y, double r, double start, double end);