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

interface/wx/image.h

@@ -318,9 +318,9 @@ const unsigned char wxIMAGE_ALPHA_OPAQUE = 0xff;
     channel with wxImage::HasAlpha. Currently the BMP, PNG, and TIFF format
     handlers have full alpha channel support for loading so if you want to use
     alpha you have to use one of these formats. If you initialize the image
-    alpha channel yourself using wxImage::SetAlpha, you should save it in PNG
-    format to avoid losing it as this is the only handler that currently
-    supports saving with alpha. 
+    alpha channel yourself using wxImage::SetAlpha, you should save it in
+    either PNG or TGA format to avoid losing it as these are the only handlers
+    that currently support saving with alpha.
 
 
     @section image_handlers Available image handlers
@@ -337,7 +337,7 @@ const unsigned char wxIMAGE_ALPHA_OPAQUE = 0xff;
     - wxPCXHandler: For loading and saving (see below).
     - wxPNMHandler: For loading and saving (see below).
     - wxTIFFHandler: For loading (including alpha support) and saving.
-    - wxTGAHandler: For loading only.
+    - wxTGAHandler: For loading and saving. Includes alpha support.
     - wxIFFHandler: For loading only.
     - wxXPMHandler: For loading and saving.
     - wxICOHandler: For loading and saving.