Fix pulsing of bitmaps in focused buttons under Windows 7.

Fix pulsing of bitmaps in focused buttons under Windows 7.

It turns out that the actual bitmap shown in this case varies between the
bitmaps at PBS_DEFAULTED and PBS_STYLUSHOT, so that it's invisible half of the
time if we don't specify the value for the latter. Do it now to fix unwanted
pulsing of the bitmap in the focused button.

Closes #15034.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73486 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775