plugin: Add GL uploader

Add "clappergluploader" module which uses "glupload" and "glcolorconvert" internally,
allowing either uploading RAW system memory into GPU with GL colorspace conversion or
importing DMABufs into GL memory and making a GdkTexture out of them at one go.

lib/gst/plugin/importers/gstclappergluploader.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2022 Rafał Dzięgiel <rafostar.github@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+#include "gstclapperglbaseimporter.h"
+
+G_BEGIN_DECLS
+
+#define GST_TYPE_CLAPPER_GL_UPLOADER (gst_clapper_gl_uploader_get_type())
+G_DECLARE_FINAL_TYPE (GstClapperGLUploader, gst_clapper_gl_uploader, GST, CLAPPER_GL_UPLOADER, GstClapperGLBaseImporter)
+
+#define GST_CLAPPER_GL_UPLOADER_CAST(obj)        ((GstClapperGLUploader *)(obj))
+
+struct _GstClapperGLUploader
+{
+  GstClapperGLBaseImporter parent;
+
+  GstGLUpload *upload;
+  GstGLColorConvert *color_convert;
+
+  GstVideoInfo pending_v_info, v_info;
+  gboolean has_pending_v_info;
+};
+
+G_END_DECLS