Discard fully transparent alpha channel when loading bitmaps.

Discard fully transparent alpha channel when loading bitmaps.

Some bitmap files declare themselves to be 32bpp, normally indicating that
they have an alpha channel, but have only zeroes in their alpha data. Hence
loading them used to create fully transparent images which wasn't the desired
effect.

Fix this by simply discarding the alpha channel entirely if it turns out that
all pixels were fully transparent.

Closes #10915.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63923 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775