libweston: Add initial output API for windowed outputs configuration

This adds new plugin-specific API for configuring outputs
on "windowed" backends, such as X11, wayland/non-fullscreen
and even headless (although, it doesn't have any windows,
its configuration is very similar). It can be used from
compositors to configure pending outputs and should be used
with previously added weston_output_set_{scale,transform}
to properly configure an output before enabling it.

It also supports creating additional outputs on the mentioned
backends.

v2:

 - Rename output-api.h to windowed-output-api.h.
 - Rename output_configure() to output_set_size().
 - Document return values.

v3:

 - Fixed copyright.
 - Noted that output name can't be NULL in
   output_create().

Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>

Makefile.am

@@ -285,6 +285,7 @@ libwestoninclude_HEADERS =			\
 	libweston/compositor-rdp.h		\
 	libweston/compositor-wayland.h		\
 	libweston/compositor-x11.h		\
+	libweston/windowed-output-api.h		\
 	libweston/plugin-registry.h		\
 	libweston/timeline-object.h		\
 	shared/matrix.h				\

libweston/windowed-output-api.h

@@ -0,0 +1,92 @@
+/*
+ * Copyright © 2016 Armin Krezović
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the
+ * next paragraph) shall be included in all copies or substantial
+ * portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#ifndef WESTON_WINDOWED_OUTPUT_API_H
+#define WESTON_WINDOWED_OUTPUT_API_H
+
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
+#include "plugin-registry.h"
+
+struct weston_compositor;
+struct weston_output;
+
+#define WESTON_WINDOWED_OUTPUT_API_NAME "weston_windowed_output_api_v1"
+
+struct weston_windowed_output_api {
+	/** Assign a given width and height to an output.
+	 *
+	 * \param output An output to be configured.
+	 * \param width  Desired width of the output.
+	 * \param height Desired height of the output.
+	 *
+	 * Returns 0 on success, -1 on failure.
+	 *
+	 * This assigns a desired width and height to a windowed
+         * output. The backend decides what should be done and applies
+	 * the desired configuration. After using this function and
+	 * generic weston_output_set_*, a windowed
+	 * output should be in a state where weston_output_enable()
+	 * can be run.
+	 */
+	int (*output_set_size)(struct weston_output *output,
+			       int width, int height);
+
+	/** Create a new windowed output.
+	 *
+	 * \param compositor The compositor instance.
+	 * \param name       Desired name for a new output.
+	 *
+	 * Returns 0 on success, -1 on failure.
+	 *
+	 * This creates a new output in the backend using this API.
+	 * After this function is ran, the created output should be
+	 * ready for configuration using the output_configure() and
+	 * weston_output_set_{scale,transform}().
+	 *
+	 * An optional name can be assigned to it, so it can be used
+	 * by compositor to configure it. It can't be NULL.
+	 */
+	int (*output_create)(struct weston_compositor *compositor,
+			     const char *name);
+};
+
+static inline const struct weston_windowed_output_api *
+weston_windowed_output_get_api(struct weston_compositor *compositor)
+{
+	const void *api;
+	api = weston_plugin_api_get(compositor, WESTON_WINDOWED_OUTPUT_API_NAME,
+				    sizeof(struct weston_windowed_output_api));
+
+	return (const struct weston_windowed_output_api *)api;
+}
+
+#ifdef  __cplusplus
+}
+#endif
+
+#endif /* WESTON_WINDOWED_OUTPUT_API_H */