Fix DrawArc() implementation in wxGtkPrinterDC.

It didn't count the angles correctly when they were equal to 90 degrees.

Also simplify the code by avoiding special cases for 90 degrees anyhow, just
call atan2() directly. And also avoid converting the angles to degrees just to
convert them back to radians immediately afterwards.

See #15609.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75064 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Don't draw the segment lines in wxGtkPrinterDC::DrawArc() unless it's filled.

The segments should only be drawn if we're filling the arc, i.e. drawing a
pie, but not if we're just drawing its outline.

Add a test of this to the printing sample.

Closes #15609.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75065 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775