Optimized unscaled yuvp9/yuvp10 -> yuvp16 conversion.

About 30% faster on 32 bit Atom, 120% faster on 64 bit Phenom2.
This is interesting because supporting P16 is easier in e.g.
OpenGL (can misuse support for any 2-component 8 bit format),
whereas supporting p9/p10 without conversion needs a texture
format with at least 14 bits actual precision.
The shiftonly == 0 case is not optimized since the code is more
complex and the speed gain less obvious.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libswscale/swscale_unscaled.c

@@ -830,7 +830,34 @@ static int planarCopyWrapper(SwsContext *c, const uint8_t *src[],
                         srcPtr  += srcStride[plane];
                     }
                 } else if (src_depth <= dst_depth) {
+                    int orig_length = length;
                     for (i = 0; i < height; i++) {
+                        if(isBE(c->srcFormat) == HAVE_BIGENDIAN &&
+                           isBE(c->dstFormat) == HAVE_BIGENDIAN &&
+                           shiftonly) {
+                             unsigned shift = dst_depth - src_depth;
+                             length = orig_length;
+#if HAVE_FAST_64BIT
+#define FAST_COPY_UP(shift) \
+    for (j = 0; j < length - 3; j += 4) { \
+        uint64_t v = AV_RN64A(srcPtr2 + j); \
+        AV_WN64A(dstPtr2 + j, v << shift); \
+    } \
+    length &= 3;
+#else
+#define FAST_COPY_UP(shift) \
+    for (j = 0; j < length - 1; j += 2) { \
+        uint32_t v = AV_RN32A(srcPtr2 + j); \
+        AV_WN32A(dstPtr2 + j, v << shift); \
+    } \
+    length &= 1;
+#endif
+                             switch (shift)
+                             {
+                             case 6: FAST_COPY_UP(6); break;
+                             case 7: FAST_COPY_UP(7); break;
+                             }
+                        }
 #define COPY_UP(r,w) \
     if(shiftonly){\
         for (j = 0; j < length; j++){ \