code like
*ptr++ = *ptr++ = *ptr++ = color;
is wrong, because there is no guarantee that ptr will be incremented
_between_ the assignment operations. it can be incremented after all
assignment operations as well. Because of this both of these are valid
implementations for a compiler:
a. assign, increment, assign, increment, assign, increment
b. assign, assign, assign, increment by 3
In case b. only the first memory location of the 3 will be modified, being
assigned 3 times the same value, while the other 2 remain unchanged.
For example egcs-2.91.66 (and possibly gcc-2.95.x too) implement this in
the second way (like in case b.)
Also the order in which the assignement is made is undefined (left to right
or right to left).
this fixed the problem we had with greyscale jpegs showing up in red,
and possibly other problems related to pseudocolor and greyscale displays.
