wpevideosrc: Clear cached SHM buffers after caps re-negotiation

Otherwise buffers not corresponding to the negotiated caps might be pushed
downstream.

Fixes #4094

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8212>

subprojects/gst-plugins-bad/ext/wpe/gstwpethreadedview.cpp

@@ -711,6 +711,37 @@ void GstWPEThreadedView::resize(int width, int height)
     });
 }
 
+void GstWPEThreadedView::clearBuffers()
+{
+  bool dispatchFrameComplete = false;
+  {
+    GMutexHolder lock(images_mutex);
+
+    if (shm.pending) {
+      auto meta = gst_buffer_get_video_meta(shm.pending);
+      if (static_cast<int>(meta->width) != wpe.width || static_cast<int>(meta->height) != wpe.height) {
+        gst_clear_buffer(&shm.pending);
+        dispatchFrameComplete = true;
+      }
+    }
+
+    if (shm.committed) {
+      auto meta = gst_buffer_get_video_meta(shm.committed);
+      if (static_cast<int>(meta->width) != wpe.width || static_cast<int>(meta->height) != wpe.height) {
+        gst_clear_buffer(&shm.committed);
+        dispatchFrameComplete = true;
+      }
+    }
+  }
+
+  if (dispatchFrameComplete) {
+    frameComplete();
+    // Wait until the next SHM buffer has been received.
+    threading.ready = false;
+    waitLoadCompletion();
+  }
+}
+
 void GstWPEThreadedView::frameComplete()
 {
     GST_TRACE("frame complete");

subprojects/gst-plugins-bad/ext/wpe/gstwpethreadedview.h

@@ -45,6 +45,7 @@ public:
     void loadData(GBytes*);
     void runJavascript(const gchar*);
     void setDrawBackground(gboolean);
+    void clearBuffers();
 
     GstEGLImage* image();
     GstBuffer* buffer();

subprojects/gst-plugins-bad/ext/wpe/gstwpevideosrc.cpp

@@ -329,6 +329,11 @@ gst_wpe_video_src_start (GstWpeVideoSrc * src)
     return FALSE;
   }
 
+  if (!created_view) {
+    GST_INFO_OBJECT (src, "Re-starting after re-negotiation, clearing cached SHM buffers");
+    src->view->clearBuffers ();
+  }
+
   GST_OBJECT_LOCK (src);
   bytes = src->bytes;
   src->bytes = NULL;