X11DRV - Fix Subtle DIB Losing Changes Bug

[Robert Shearman]

Hi,

This bug was very subtle. It only occurs after a specific sequence of 
changes of state in the DIB state machine and results in the most recent 
set of changes being lost. It is caused by DIB_Status_GdiMod not storing 
its previous status. Here is the sequence that triggers the bug:
1) Application reads from DIB bits. (dib->status = DIB_Status_InSync, 
dib->p_status = ?)
2) Applications does some GDI operation that causes a change to the DIB 
(dib->status = DIB_Status_GdiMod, dib->p_status = DIB_Status_InSync).
3) Application does another GDI operation that doesn't change the DIB 
(dib->status = DIB_Status_GdiMod, dib->p_status = DIB_Status_InSync).
3a) Because the operation didn't change the DIB, it reverts to its 
previously stored state (dib->status = DIB_Status_InSync, dib->p_status 
= DIB_Status_InSync).
Therefore, any changes in (2) become lost.
This patch fixes the above case.

Changelog:
When changing from DIB_Status_GdiMod to DIB_Status_GdiMod, we have to 
set the previous status to DIB_Status_GdiMod, otherwise a non-committing 
unlock could incorrectly revert the DIB to another state, rather than 
the correct DIB_Status_GdiMod.
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