[AD] asm blitter patch

["Eric Botcazou" <ebotcazou@xxxxxxxxxx>]

After further investigations, it turned out that the problem in
video->memory blit operations under Windows, although related to the
locking mechanism, actually originates in a glitch in the asm blitters...
I introduced three months ago while fixing the "%es not restored quickly
enough" bug.

In my defence ;-)
- the macro UNWRITE_BANK() was a little misleading for the source bitmap
that is only read,
- a little comment wouldn't have been useless in the masked_blit() routines
to explain the apparent inconsistency.

Tested under Windows only.
Could someone test it on another platform and commit it ?

---
Eric Botcazou
ebotcazou@xxxxxxxxxx

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