This commit changes the command generation mechanism. Instead of
creating a command on the stack then calling azrp_queue_command() which
does a needless copy, the new function azrp_new_command() allocates
directly inside the command buffer, avoiding the copy.
This concept is pushed further with the introduction of finer-grained
functions azrp_alloc_command(), azrp_finalize_command() and
azrp_instantiate_command(), which split this process in three steps.
This allows generating commands of variadic size in-place in the
command buffer as well as using non-interval fragment sets.
Lephenixnoir