app: add trivial cast macros

Add trivial cast macros for appsrc and appsink. Mark them as being since 0.10.23
and add the macros to the standard macros in the docs.

Fixes #579130

docs/libs/gst-plugins-base-libs-sections.txt

@@ -24,6 +24,7 @@ gst_app_src_end_of_stream
 <SUBSECTION Standard>
 GstAppSrcClass
 GST_APP_SRC
+GST_APP_SRC_CAST
 GST_IS_APP_SRC
 GST_TYPE_APP_SRC
 gst_app_src_get_type
@@ -65,6 +66,7 @@ gst_app_sink_set_callbacks
 GstAppSinkPrivate
 GstAppSinkClass
 GST_APP_SINK
+GST_APP_SINK_CAST
 GST_IS_APP_SINK
 GST_TYPE_APP_SINK
 gst_app_sink_get_type

gst-libs/gst/app/gstappsink.h

@@ -35,6 +35,9 @@ G_BEGIN_DECLS
   (G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_APP_SINK))
 #define GST_IS_APP_SINK_CLASS(klass) \
   (G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_APP_SINK))
+/* Since 0.10.23 */
+#define GST_APP_SINK_CAST(obj) \
+  ((GstAppSink*)(obj))
 
 typedef struct _GstAppSink GstAppSink;
 typedef struct _GstAppSinkClass GstAppSinkClass;

gst-libs/gst/app/gstappsrc.h

@@ -35,6 +35,9 @@ G_BEGIN_DECLS
   (G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_APP_SRC))
 #define GST_IS_APP_SRC_CLASS(klass) \
   (G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_APP_SRC))
+/* Since 0.10.23 */
+#define GST_APP_SRC_CAST(obj) \
+  ((GstAppSrc*)(obj))
 
 typedef struct _GstAppSrc GstAppSrc;
 typedef struct _GstAppSrcClass GstAppSrcClass;