cogl-material-arbfp: Use separate buffers when calling g_ascii_dtostr

cogl-material-arbfp: Use separate buffers when calling g_ascii_dtostr

g_ascii_dtostr was being used in four separate arguments to
g_string_append_printf but all invocations of it were using the same
buffer. This would end up with all of the arguments having the same
value which would depend on whichever order the compiler evaluates
them in. This patches changes it to use a multi-dimensional array and
a loop to fill in the separate buffers.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2219