winemac: Don't invalidate the window shadow on every draw if it's merely shaped and not color-keyed or using per-pixel alpha.

This avoids flickering and tearing on some versions of OS X during frequent
redrawing in a shaped window, such as when scrolling a document in Word 2007.

Since we aren't guaranteed that the window surface has updated bits for us to
draw, we mark the whole content view as needing redisplay and draw the window's
shape in the background color on the first -drawRect: after the shape change.