[bug #22247] Pre-multiplied alpha values for bitmaps

[Fred Kiefer]

Follow-up Comment #2, bug #22247 (project gnustep):

Moved the unpre-multiplication in the art backend one step down. The code was
already there for one format (RGB, 8 bit without clipping) and is now used in
all cases.
A next step would be to remove this unpre-multiply completely and adopt the
image processing code to cope with pre-multiplied colours. This should be
faster as this was the whole idea behind pre-multiplying.

Another thing that is still needed is the conversion of all bitmaps into the
pre-multiplied format, which is now consistently expected by all backends.

And just for the record: I checked JPEG cannot handle transparency, so there
is nothing to do here and although GIF handles transparency it allows only
alpha values of 0 and 255, so no pre-multiplication is needed there.

The othe formats and the windows backecd still need to be checked.

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