Make pixman an optional dependency
Additional information:
Only these files use pixman functions (excluding tests, of course):
```
meson.build
contrib/vhost-user-gpu/vhost-user-gpu.c
contrib/vhost-user-gpu/meson.build
chardev/meson.build
include/ui/spice-display.h
include/ui/sdl2.h
include/ui/gtk.h
include/ui/qemu-pixman.h
include/ui/console.h
include/hw/display/xlnx_dp.h
include/hw/virtio/virtio-gpu.h
include/hw/virtio/virtio-gpu-pixman.h
hw/display/vga.c
hw/display/ramfb.c
hw/display/vhost-user-gpu.c
hw/display/virtio-gpu-gl.c
hw/display/virtio-gpu-udmabuf.c
hw/display/xenfb.c
hw/display/ati_2d.c
hw/display/meson.build
hw/display/vmware_vga.c
hw/display/qxl-render.c
hw/display/xlnx_dp.c
hw/display/bochs-display.c
hw/display/sm501.c
hw/display/virtio-gpu.c
hw/vfio/display.c
hw/s390x/meson.build
ui/cocoa.m
ui/console-gl.c
ui/vnc.c
ui/qemu-pixman.c
ui/gtk.c
ui/console.c
ui/trace-events
ui/meson.build
ui/dbus-listener.c
ui/vnc-enc-tight.c
ui/vnc.h
ui/spice-display.c
ui/dbus-display1.xml
ui/sdl2-2d.c
```

This code in `meson.build` always require **pixman** for building system emulators:
```meson
pixman = not_found
if have_system or have_tools
  pixman = dependency('pixman-1', required: have_system, version:'>=0.21.8',
                      method: 'pkg-config', kwargs: static_kwargs)
endif
```
https://gitlab.com/qemu-project/qemu/-/blob/master/meson.build#L520

Most of the code could work without it.