[AD] possible speed improvements on current allegro blenders

["Javier Gonzalez" <Xavier_Gonzalez@xxxxxxxxxx>]

Hello, I just noticed allegro doesn't take adventage of special cases in the
blenders to speed up this operation
for example, in the trans blender, if the alpha is 255 (one of the most used
cases) it could avoid reading the back pixel and simply returning the pixel
to be put
Same thing if the alpha is 0, it could simply avoid reading and do not
putpix anything (or if that can't be done, simply return the value of the
getpix)

Well, what do you think, that's it, thanks :)