Optimize macDisplayColorSpace

On 10.9, getting the display based on the rect is slow(er).

Use a window -> color space cache instead of a
display -> color space cache.

Change-Id: If40a6d089019a54ceb724b8007adaffea89c96fb
Task-number: QTBUG-34358
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>