ui/console: use QEMU_PIXMAN_COLOR helpers

QEMU_RGB macro is actually defining a pixman color. Make this explicit
in the macro name. Move it to qemu-pixman.h so it can be used elsewhere,
as done in the following patch. Finally, define
QEMU_PIXMAN_COLOR_{BLACK,GRAY}, to avoid need to look up the VGA color
table from the QemuConsole placeholder surface rendering.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230830093843.3531473-37-marcandre.lureau@redhat.com>