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

docs/changes.txt

@@ -573,6 +573,10 @@ All (GUI):
 
 - Fix wxListEvent::GetData() in LLP64 builds.
 
+wxGTK:
+
+- Fix DrawArc() when printing.
+
 wxMSW:
 
 - Optional better handling of SAFEARRAYs in wxAutomationObject (PB).