lavfi/vf_stack_vaapi: factor out the common code for stack setting

The common code will be used in QSV based stack filters.
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavfilter/stack_internal.h

+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVFILTER_STACK_INTERNAL_H
+#define AVFILTER_STACK_INTERNAL_H
+
+enum {
+    STACK_H = 0,
+    STACK_V = 1,
+    STACK_X = 2
+};
+
+typedef struct StackItemRegion {
+    int x;
+    int y;
+    int width;
+    int height;
+} StackItemRegion;
+
+typedef struct StackBaseContext {
+    HWContext hwctx; /**< must be the first field */
+
+    FFFrameSync fs;
+    int mode;
+    uint8_t fillcolor[4];
+    int fillcolor_enable;
+    StackItemRegion *regions;
+
+    /* Options */
+    int nb_inputs;
+    int shortest;
+    int tile_width;
+    int tile_height;
+    int nb_grid_columns;
+    int nb_grid_rows;
+    char *layout;
+    char *fillcolor_str;
+} StackBaseContext;
+
+static int config_comm_output(AVFilterLink *outlink);
+static int stack_init(AVFilterContext *avctx);
+static av_cold void stack_uninit(AVFilterContext *avctx);
+static int stack_activate(AVFilterContext *avctx);
+
+#endif /* AVFILTER_STACK_INTERNAL_H */