Use a single global StringFormat in all wxMSW wxGraphicsContext code.

Use a single global StringFormat in all wxMSW wxGraphicsContext code.

Instead of creating StringFormat in DrawText() and Get[Partial]TextExtent[s],
create it once and simply use it from both functions.

This might be slightly more efficient as we don't waste time recreating it but
the main advantage is that it ensures that these functions use the same string
format and no discrepancies between them are possible.

See #14537.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72441 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775