Change VIEWPORT_EXTENT constant in wxMSW wxDC code to be a power of two.

Change VIEWPORT_EXTENT constant in wxMSW wxDC code to be a power of two.

Notice that this commit doesn't really change anything, in particular it
doesn't correct zooming for zoom factors that are power of 2 (which was broken
before as ticket #11669 showed), this was already done by the previous commit.
This one simply makes multiplying by VIEWPORT_EXTENT a tiny bit more efficient.

See #9554.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67624 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775