* 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.