Add avpriv_dsputil_init() and use it in lavfi where relevant.

dsputil_init() is deprecated and not meant to be exported.
ff_dsputil_init() is internal to libavcodec and thus can not be used.
avpriv_dsputil_init() is the version shared between libraries.

This commit fixes 3 unjustified libavfilter deprecated warnings.

libavcodec/dsputil.c

@@ -2928,3 +2928,8 @@ av_cold void dsputil_init(DSPContext* c, AVCodecContext *avctx)
 {
     ff_dsputil_init(c, avctx);
 }
+
+av_cold void avpriv_dsputil_init(DSPContext *c, AVCodecContext *avctx)
+{
+    ff_dsputil_init(c, avctx);
+}

libavcodec/dsputil.h

@@ -315,6 +315,7 @@ typedef struct DSPContext {
 
 void ff_dsputil_static_init(void);
 void ff_dsputil_init(DSPContext* p, AVCodecContext *avctx);
+void avpriv_dsputil_init(DSPContext* p, AVCodecContext *avctx);
 attribute_deprecated void dsputil_init(DSPContext* c, AVCodecContext *avctx);
 
 int ff_check_alignment(void);

libavfilter/f_select.c

@@ -199,7 +199,7 @@ static int config_input(AVFilterLink *inlink)
         select->avctx = avcodec_alloc_context3(NULL);
         if (!select->avctx)
             return AVERROR(ENOMEM);
-        dsputil_init(&select->c, select->avctx);
+        avpriv_dsputil_init(&select->c, select->avctx);
     }
 #endif
     return 0;

libavfilter/vf_deshake.c

@@ -407,7 +407,7 @@ static int config_props(AVFilterLink *link)
     deshake->last.zoom = 0;
 
     deshake->avctx = avcodec_alloc_context3(NULL);
-    dsputil_init(&deshake->c, deshake->avctx);
+    avpriv_dsputil_init(&deshake->c, deshake->avctx);
 
     return 0;
 }

libavfilter/vf_mpdecimate.c

@@ -139,7 +139,7 @@ static av_cold int init(AVFilterContext *ctx)
     decimate->avctx = avcodec_alloc_context3(NULL);
     if (!decimate->avctx)
         return AVERROR(ENOMEM);
-    dsputil_init(&decimate->dspctx, decimate->avctx);
+    avpriv_dsputil_init(&decimate->dspctx, decimate->avctx);
 
     return 0;
 }