lavfi/vf_vpp_qsv: add vpp_preinit callback

Set the expected default value for options in this callback, hence we
have the right values even if these options are not included in the
option arrray.

This is in preparation for reusing the code for other QSV filters.

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavfilter/vf_vpp_qsv.c

@@ -259,6 +259,19 @@ release:
     return ret;
 }
 
+static av_cold int vpp_preinit(AVFilterContext *ctx)
+{
+    VPPContext  *vpp  = ctx->priv;
+    /* For AV_OPT_TYPE_STRING options, NULL is handled in other way so
+     * we needn't set default value here
+     */
+    vpp->saturation = 1.0;
+    vpp->contrast = 1.0;
+    vpp->transpose = -1;
+
+    return 0;
+}
+
 static av_cold int vpp_init(AVFilterContext *ctx)
 {
     VPPContext  *vpp  = ctx->priv;
@@ -683,6 +696,7 @@ const AVFilter ff_vf_vpp_qsv = {
     .name          = "vpp_qsv",
     .description   = NULL_IF_CONFIG_SMALL("Quick Sync Video VPP."),
     .priv_size     = sizeof(VPPContext),
+    .preinit       = vpp_preinit,
     .init          = vpp_init,
     .uninit        = vpp_uninit,
     FILTER_INPUTS(vpp_inputs),