[AD] pink color conversion blit bug

["Elias Pschernig" <eliaspschernig@xxxxxxxxxx>]

I made a small patch for a bug I always had to work around in my
programs: (The bug occures very rarely and is hard to notice, but
anyway, a bug is a bug) If you use blit to convert between color
depths, this doesn't correctly work if the bitmap contains transparent
or pink areas. I.e. pink wrongly gets transparent for some pixels.
Here  is a little test program which shows the bug:
http://www.allefant.f2s.com/allegro/pink.c

Attached is a patch to fix this. But because blit might be used in
programs without transparency, it is only invoked when using the newly
added COLORCONV_TRANS_CARE flag for color conversion.

In case the patch is accepted I guess I should add a note about
COLORCONV_TRANS_CARE to the documentation as well..

Elias Pschernig

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