[Improved] Arrow custom drawing for button widgets

[inmarket]

Previously it was difficult to get a clean arrow head on a button when using the arrow custom draws.

This is now much simpler as we have added support for drawing arrow heads with 45° angles. This is now the default. The old method can still be achieved by altering the drawing parameters at the top of gwin_button.c

An arrow symmetry bug has also been fixed.

This post is related to the issue: viewtopic.php?f=9&t=154