Make Pixman an optional dependency

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# gpg: Signature made Tue 07 Nov 2023 18:14:49 HKT
# gpg:                using RSA key 87A9BD933F87C606D276F62DDAE8E10975969CE5
# gpg:                issuer "marcandre.lureau@redhat.com"
# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
# gpg:                 aka "Marc-André Lureau <marcandre.lureau@gmail.com>" [full]
# Primary key fingerprint: 87A9 BD93 3F87 C606 D276  F62D DAE8 E109 7596 9CE5

* tag 'pixman-pull-request' of https://gitlab.com/marcandre.lureau/qemu

: (25 commits)
  build-sys: make pixman actually optional
  hw/display/ati: allow compiling without PIXMAN
  hw/mips: FULOONG depends on VT82C686
  hw/sm501: allow compiling without PIXMAN
  hw/arm: XLNX_VERSAL depends on XLNX_CSU_DMA
  arm/kconfig: XLNX_ZYNQMP_ARM depends on PIXMAN
  ui/dbus: do not require PIXMAN
  ui/gtk: -display gtk requires PIXMAN
  ui/spice: SPICE/QXL requires PIXMAN
  ui/vnc: VNC requires PIXMAN
  ui/gl: opengl doesn't require PIXMAN
  vhost-user-gpu: skip VHOST_USER_GPU_UPDATE when !PIXMAN
  ui/console: when PIXMAN is unavailable, don't draw placeholder msg
  virtio-gpu: replace PIXMAN for region/rect test
  qmp/hmp: disable screendump if PIXMAN is missing
  ui/vc: console-vc requires PIXMAN
  ui/console: allow to override the default VC
  vl: move display early init before default devices
  vl: simplify display_remote logic
  qemu-options: define -vnc only #ifdef CONFIG_VNC
  ...

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>