LibGfx: Implement nicer antialiased lines

This is not any 'proper' algorithm, this was just a shower thought
idea. There probably is a better algorithm to achieve the same effect
out there, if someone knows of one please replace this code :^).

This works by rendering the line a scanline at a time, which avoids
repainting over any pixel on the line (so opacity now works with AA
lines). This generally seems to achieve a much nicer looking line.

I've not done any proper benchmarking of this, but some little messing
around showed that this new implementation was a little faster than
the old one too, so that's a nice little bonus.

With the inclusion of a few minor hacks this also goes a surprisingly
far way in improving our SVG rendering too (for both filled and stroked
paths). :^)

Userland/Libraries/LibGfx/AntiAliasingPainter.h

@@ -82,7 +82,7 @@ private:
         Full,
     };
     template<AntiAliasPolicy policy>
-    void draw_anti_aliased_line(FloatPoint const&, FloatPoint const&, Color, float thickness, Painter::LineStyle style, Color alternate_color);
+    void draw_anti_aliased_line(FloatPoint, FloatPoint, Color, float thickness, Painter::LineStyle style, Color alternate_color);
     void stroke_segment_intersection(FloatPoint const& current_line_a, FloatPoint const& current_line_b, FloatLine const& previous_line, Color, float thickness);
     FloatQuad build_rotated_rectangle(FloatPoint const& direction, float width);