Handle transparency to the best of our ability in wxImageList

Don't take the value of "mask" parameter of wxImageList constructor too
prescriptively, it predates support for alpha in wxWidgets by many years
and was never meant to actually suppress using it.

Instead, do the best thing we can in all cases, i.e. use alpha if it's
specified and supported and use mask otherwise. But only create the mask
from light grey colour if we have nothing else if "mask" is true in
wxImageList constructor, as this is a potentially destructive action
that must not be performed if the user has explicitly decided to set
this parameter to false.

Incidentally fix handling of alpha with comctl32.dll v5 (which is used
in the absence of any manifest) by converting it to a mask in this case:
this is not ideal, but better than just using black background as it
happened before, and restores pre-3.1.5 behaviour.

Also simplify the generic version which just has to create the default
mask if necessary and doesn't have to do anything at all in all the
other cases.

Note that these changes required relaxing some of the existing unit
tests as wxMSW implementation now can add alpha channel to the bitmaps
that didn't have it -- but this is a more useful behaviour, and so it
makes more sense to adapt the tests to it rather than doing a less
useful thing just to conform to the tests.

This commit is best viewed with git --color-moved
--color-moved-ws=ignore-all-spac options.

Closes #22349.

interface/wx/imaglist.h

@@ -28,8 +28,8 @@ enum
     @class wxImageList
 
     A wxImageList contains a list of images, which are stored in an unspecified
-    form. Images can have masks for transparent drawing, and can be made from a
-    variety of sources including bitmaps and icons.
+    form. Images can use alpha channel or masks for transparent drawing, and
+    can be made from a variety of sources including bitmaps and icons.
 
     wxImageList is used principally in conjunction with wxTreeCtrl and
     wxListCtrl classes.
@@ -62,7 +62,12 @@ public:
         @param height
             Height of the images in the list.
         @param mask
-            @true if masks should be created for all images.
+            If @true, all images will have masks, with the mask being created
+            from the light grey pixels if not specified otherwise, i.e. if the
+            image doesn't have neither alpha channel nor mask and no mask is
+            explicitly specified when adding it. Note that if an image does
+            have alpha channel or mask, it will always be used, whether this
+            parameter is @true or @false.
         @param initialCount
             The initial size of the list.