pipewire: Introduce well known tags

Tags are meant to be high level properties or hints of a stream, e.g.
scaling information.

Tags are split into 'input' tags, set by consumers which typically would
be hints about preferences, or 'output' tags which contain high level
information about a stream, set by producers.

Start by defining two tags:

  * `pipewire.output.stream.scale`, which could be the HiDPI scale of a
    streamed window, or the HiDPI scale of a streamed monitor.

  * `pipewire.input.stream.preferred-scale`, which can be used by a
    virtual monitor consumer to hint about what the preferred scale of
    the corresponding virtual monitor should be.

src/pipewire/meson.build

@@ -39,6 +39,7 @@ pipewire_headers = [
   'proxy.h',
   'resource.h',
   'stream.h',
+  'tags.h',
   'thread.h',
   'thread-loop.h',
   'timer-queue.h',

src/pipewire/tags.h

@@ -0,0 +1,48 @@
+/* PipeWire */
+/* SPDX-FileCopyrightText: Copyright © 2026 Red Hat */
+/* SPDX-License-Identifier: MIT */
+
+#ifndef PIPEWIRE_TAGS_H
+#define PIPEWIRE_TAGS_H
+
+#include <pipewire/utils.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \defgroup pw_stream_tags Well known stream tags
+ *
+ * A collection of tags that can be used for knows dynamic stream properties.
+ *
+ * \addtogroup pw_stream_tags
+ * \{
+ */
+
+/**
+ * Output tags, set by stream producers.
+ */
+
+/**< Stream contents should be considered to be scaled according to the
+ * specified floating point number. */
+#define PW_STREAM_OUTPUT_TAG_SCALE			"pipewire.output.stream.scale"
+
+
+/**
+ * Input tags, set by stream consumers.
+ */
+
+/**< Stream consumer prefers the producer produces content with the scale
+ * specified with this floating point number.
+*/
+#define PW_STREAM_INPUT_TAG_PREFERRED_SCALE		"pipewire.input.stream.preferred-scale"
+
+/** \}
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* PIPEWIRE_TAGS_H */