Implement context-sensitive alpha blending for 24 or 32-bit images

[sdlime]

This is sort of the next step beyond Sean's work with bug 926. The problem with
that approach is that using TRANSPARENCY ALPHA precludes using the general layer
transparency. It is easy enough to change the code in mapgd.h so that alpha
blending is turned on with 24-bit symbols and in that case you no longer need
the ALPHA setting. I *think* our approach should be to only turn on blending
when necessary and turn off again when done. That actually simplifies the code a
bit since you don't have to save the blending state when entering the drawing or
labeling functions.

My initial tests work ok although I have to try many more symbol output image
combinations. Ideas how to automate?

So now I can draw markers with alpha channels and also make them even more
transparent. I did uncover a bug in in our image merging code that doesn't seem
to deal with alpha channels quite right, but when fixed the results are great.

Steve

[tbonfort]

we don't use transparency alpha anymore