Added saving support to TGA image handler.

Supports saving 24-bit and 32-bit (RGB with alpha).
Updated image unit test to verify the alpha channel of saved TGA images. Also removed a condition skipping a test which only was in place for TGA (formerly its saving handler would do nothing yet say saving was succesful).

See also #7661.



git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66485 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -449,6 +449,7 @@ All (GUI):
 - Return bool, not void, from wxImage::ConvertAlphaToMask() (troelsk).
 - Fixed resizing columns in wxGrid when they were reordered.
 - Added wxImage::Rotate180() (Jeff Tupper).
+- Added support for saving TGA files.
 
 GTK: