[THEME] Load UXTHEME.DLL even if COMCTL32.DLL is old.

We load UXTHEME.DLL even if COMCTL32.DLL older then 6.0 is in use.
This allows us to use buffered painting and other stuff from the
UXTHEME.DLL even when app is not themed.

(We still disable themed painting by making mcOpenThemeData(Ex)() return
NULL.)

Furthermore on Windows XP we do not attempt to GetProcAddress() of many
functions added in later Windows. THis saves quite many CPU cycles with
a single branch.
[DOUBLEBUFFER] Fix double-buffering without UXTHEME.DLL.

We did not remember the target HDC so nothing was actually blit to the
screen.