Testing for drawing fixes r30523

[Quentin Mathé]

Hi,

I just committed a patch I have been working on for a while. See bug  
report https://savannah.gnu.org/bugs/?27782

This should solve various differences between GNUstep vs Cocoa image  
drawing when Cairo is used, especially with -[NSImage  
drawInRect:fromRect:operation:fraction:] or similar.

For the Cairo backend, this method should now give the same result  
than Cocoa when the destination is flipped, rotated or scaled. The  
scaling between the inRect and the fromRect should also work  
correctly. Various border cases such as passing NSZeroRect are now  
handled in a way that matches Cocoa.
The performance should also be much better when you draw a small  
source rect from a big image, since we only manipulate the source rect  
area rather than the whole image now. We also do not create an  
intermediate image to apply the graphics context transform, we just  
delegate that to Cairo, so this should improve performance a bit too.

Cairo also had various issues with -compositeGStateXXX and - 
DPSimageXXX. This means NSImage composite/dissolve methods now behave  
correctly, NSCopyBits too and -[NSView scrollRect:by:] should work in  
non-flipped coordinates unlike previously.

The Cairo backend unlike other backends should now match the Cocoa  
image drawing behavior exactly. This should be visible if you play  
with NSAffineTransform a lot. For example, in a flipped view, concat a  
rotation, a scaling and a manual flip transform, then draw an image  
with by scaling it with inRect and fromRect.

Art, Xlib and Winlib backends shouldn't have changed much since they  
still use the old code path. This code path is much more complex than  
the Cairo code path that delegates as much as possible to the backend  
rather than trying to handle things like rotation on the Gui side.

I tested Cairo, Art and Xlib, they work fine for me with GSTest, Gorm  
and various Étoilé applications. I haven't tested the Winlib backend,  
so I'm interested to know if it still works correctly.
I'll commit the test application I have been using to compare Cocoa vs  
GNUstep behavior soon.

Although I tried to simplify the code as much as possible, there are  
still some cleaning to do and also things to be worked out. e.g.  
NSCopyBits doesn't behave the same on GNUstep and Cocoa when you give  
it a view gstate.

Cheers,
Quentin.