Improved detection of alpha channels in TIFF images.

Improved detection of alpha channels in TIFF images.

Some TIFF images are not properly formed, for example having an extra channel marked as being unspecified data while they should be treated as being an alpha channel. Detect some of those cases so that these TIFF images now will have alpha.

Applied patch by gmeeker. Closes #12874.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66717 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775