WIN unify deferred message handling

Moves the common code into ProcessOrDeferMessage.

All callers (try to) aquire the mutex before getting the
WinSalFrame pointer, except for ImplHandleDeferredPaintMsg.

This was probably an oversight, so this reverses the order in this
function, so the GetWindowPtr is now always protected by the Solar
mutex.

Change-Id: Icb8f34f7e8716f0faa90a95903b1681f7489aed2