pixman: add submodule

Add pixman submodule as fallback for old distros.
Picking version 0.18.4.  This is shipped by rhel6
and also the minimum version needed by spice so this
should serve well as baseline.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "roms/sgabios"]
 	path = roms/sgabios
 	url = git://git.qemu.org/sgabios.git
+[submodule "pixman"]
+	path = pixman
+	url = git://anongit.freedesktop.org/pixman

pixman @ 97336fad

+Subproject commit 97336fad32acf802003855cd8bd6477fa49a12e3