Update spice-protocol to 0.12.6
Fix min gcc version for __attribute__(format)

We currently use it only on gcc 4.5 or newer, but it was actually
introduced much earlier than that. It's documented in gcc 2.95.3
manual:
http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC84
and glib uses starting from gcc 2.2.5.
This commit uses the same minimum version as glib.

This was causing warnings on RHEL6 systems which have gcc 4.4.7
proto: comment future surface flags usage
proto: add fake last message in base channel

Make it explicit that 100 is the last value of the base channel
messages. This allows clients to use the generated enum value too.
canvas: use precomputed revers_bits

Thos function shows up in some profiling results, it seems we can
trivially replace it with a precomputed array of 256bytes.

before:
     5.66%           691  lt-spicy-stats
libspice-client-glib-2.0.so.8.4.0  [.] revers_bits

after:
     0.53%            64  lt-spicy-stats
libspice-client-glib-2.0.so.8.4.0  [.] revers_bits
quic: compile with constant bpp

This simplifies a little bit function calling, and allows for compiler
to potentially specialize and optimize a bit better each version.
quic: precompute golomb codes

We can avoid repetitive computation by using two precomputed array, of
8k each.

before:
     1.79%  lt-spicy-stats  libspice-client-glib-2.0.so.8.4.0  [.]
     golomb_code_len_8bpc

after:
     0.79%  lt-spicy-stats  libspice-client-glib-2.0.so.8.4.0  [.]
     golomb_code_len_8bpc
Update spice-protocol