ASFML v2.6.0: update to latest CSFML version

Bringing ASFML inline with CSFML 2.6! 🎉

• SFML feature implementations:
  • Sf.Graphics.Font.setSmooth / Sf.Graphics.Font.isSmooth (SFML/CSFML#186)
  • Sf.Network.Packet.getReadPosition (SFML/CSFML#186)
  • Sf.Graphics.Blendmode.sfBlendMin & sfBlendMax (SFML/CSFML#191)
  • Sf.Graphics.Rect.getPosition & Sf.Graphics.Rect.getSize (SFML/CSFML#190)
  • Document MP3 support (SFML/CSFML#193)
  • Add directional arrow resize cursors (SFML/CSFML#194)
  • Sf.Graphics.Font.getBoldKerning (SFML/CSFML#192)
  • Add Sf.Window.Vulkan functions (SFML/CSFML#198, SFML/CSFML#212)
  • Add scancode support (SFML/CSFML#197)
  • Sf.Graphics.RenderTexture.isSrgb / Sf.Graphics.RenderWindow.isSrgb (SFML/CSFML#196)
  • Sf.Graphics.Image.saveToMemory with sfBuffer (SFML/CSFML#205)
  • Sf.Window.WindowBase (SFML/CSFML#207)

For the complete note of the CSFML 2.6 release see https://github.com/SFML/CSFML/releases/tag/2.6.0

Tests in ASFML can now directly executed from the repository: cd tests && make all run.

Known problems

Sf.Window.Vulkan.getGraphicsRequiredInstanceExtensions cannot be called yet due to a problem in CSFML, see:
SFML/CSFML#227

Full Changelog: v2.5.5...v2.6.0