avcodec/webp: add optimization: use local palette with extra padding

for big enough pictures.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/webp.c

@@ -1061,15 +1061,31 @@ static int apply_color_indexing_transform(WebPContext *s)
         av_free(line);
     }
 
-    for (y = 0; y < img->frame->height; y++) {
-        for (x = 0; x < img->frame->width; x++) {
-            p = GET_PIXEL(img->frame, x, y);
-            i = p[2];
-            if (i >= pal->frame->width) {
-                AV_WB32(p, 0x00000000);
-            } else {
-                const uint8_t *pi = GET_PIXEL(pal->frame, i, 0);
-                AV_COPY32(p, pi);
+    // switch to local palette if it's worth initializing it
+    if (img->frame->height * img->frame->width > 300) {
+        uint8_t palette[256 * 4];
+        const int size = pal->frame->width * 4;
+        memcpy(palette, GET_PIXEL(pal->frame, 0, 0), size);   // copy palette
+        // set extra entries to transparent black
+        memset(palette + size, 0, 256 * 4 - size);
+        for (y = 0; y < img->frame->height; y++) {
+            for (x = 0; x < img->frame->width; x++) {
+                p = GET_PIXEL(img->frame, x, y);
+                i = p[2];
+                AV_COPY32(p, &palette[i * 4]);
+            }
+        }
+    } else {
+        for (y = 0; y < img->frame->height; y++) {
+            for (x = 0; x < img->frame->width; x++) {
+                p = GET_PIXEL(img->frame, x, y);
+                i = p[2];
+                if (i >= pal->frame->width) {
+                    AV_WB32(p, 0x00000000);
+                } else {
+                    const uint8_t *pi = GET_PIXEL(pal->frame, i, 0);
+                    AV_COPY32(p, pi);
+                }
             }
         }
     }