avcodec/dxvenc: Fix data races with slice threading

The old code set a common struct from each thread;
this only "worked" (but is still UB) because the values
written are the same for each thread.
Fix this by moving the assignments to the main thread.

(This also avoids casting const away from a const AVFrame*.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/dxvenc.c

@@ -130,25 +130,6 @@ typedef struct DXVEncContext {
     HTEntry lut_lookback_ht[LOOKBACK_HT_ELEMS];
 } DXVEncContext;
 
-static int compress_texture_thread(AVCodecContext *avctx, void *arg,
-                                   int slice, int thread_nb)
-{
-    DXVEncContext *ctx = avctx->priv_data;
-    AVFrame *frame = arg;
-
-    if (ctx->enc.tex_funct) {
-        ctx->enc.tex_data.out = ctx->tex_data;
-        ctx->enc.frame_data.in = frame->data[0];
-        ctx->enc.stride = frame->linesize[0];
-        return ff_texturedsp_compress_thread(avctx, &ctx->enc, slice, thread_nb);
-    } else {
-        /* unimplemented: YCoCg formats */
-        return AVERROR_INVALIDDATA;
-    }
-
-    return 0;
-}
-
 /* Converts an index offset value to a 2-bit opcode and pushes it to a stream.
  * Inverse of CHECKPOINT in dxv.c.  */
 #define PUSH_OP(x)                                                            \
@@ -252,7 +233,15 @@ static int dxv_encode(AVCodecContext *avctx, AVPacket *pkt,
     if (ret < 0)
         return ret;
 
-    avctx->execute2(avctx, compress_texture_thread, (void*)frame, NULL, ctx->enc.slice_count);
+    if (ctx->enc.tex_funct) {
+        ctx->enc.tex_data.out = ctx->tex_data;
+        ctx->enc.frame_data.in = frame->data[0];
+        ctx->enc.stride = frame->linesize[0];
+        avctx->execute2(avctx, ff_texturedsp_compress_thread, &ctx->enc, NULL, ctx->enc.slice_count);
+    } else {
+        /* unimplemented: YCoCg formats */
+        return AVERROR_INVALIDDATA;
+    }
 
     bytestream2_init_writer(pbc, pkt->data, pkt->size);