implementation of [NSWindow cacheImageInRect:]

[Benhur Stein]

Hi,

The attached patch implements three missing methods in NSWindow:
- cacheImageInRect:
- restoreCachedImage
- discardCachedImage
It adds 2 new instance variables to NSWindow, forcing the recompilation of 
everything.

Apple's documentation on this is not very clear, in many places it seems that 
more than one
rectangle could be cached and restored. In this implementation, only the last 
rectangle is
kept. There is this phrase in the documentation: "Only the last cached rectangle is 
remembered, and can be restored."

Benhur Stein

<NSWindow.h.diff><NSWindow.m.diff>

NSWindow.h.diff
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NSWindow.m.diff
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