dot: improve option initialization
* spot/twaalgos/dot.cc: Rearrange options to speed up their initialization and avoid an "uninitialized read" error from valgrind when compiling with clang-3.9. The uninitialized read is still a bit misterious to me; valgrind was complaining about opt_shape_ who is actually initialized in the code. However looking into the assembly code generated revealed that all consecutive 0/false values were initialized together, so this patch reorganize the options to encourage that. Also the palette was copied over for each call to print_dot(), so this is now declared statically.