wined3d: Change calls of GetDC() to GetDCEx() with DCX_CACHE so the HDC is not shared with other threads.

Windows created by the app may use the CS_CLASSDC or CS_OWNDC class styles.  In
that case, GetDC() would return the same HDC to all callers.  It's not safe,
though, for multiple threads to use the same HDC without synchronization.  The
app may be using that HDC from multiple threads and using some synchronization
scheme to make that safe, but wined3d is not able to cooperate in such a scheme.

Using GetDCEx() with DCX_CACHE ensures that wined3d gets an independent HDC.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>