lavc/exrdsp: unroll predictor

With explicit unrolling, we can skip half of the sign bit flips, and
the compiler is then better able to optimise the scalar loop:

predictor_c: 31376.0 (before)
predictor_c: 23703.0 (after)

libavcodec/exrdsp.c

@@ -40,10 +40,20 @@ static void reorder_pixels_scalar(uint8_t *dst, const uint8_t *src, ptrdiff_t si
 
 static void predictor_scalar(uint8_t *src, ptrdiff_t size)
 {
-    ptrdiff_t i;
+    /* Unrolled: `src[i + 1] += src[i] - 128;` */
+    if ((size & 1) == 0) {
+        src[1] += src[0] ^ 0x80;
+        src++;
+        size--;
+    }
+
+    for (ptrdiff_t i = 1; i < size; i += 2) {
+        uint8_t a = src[i] + src[i - 1];
 
-    for (i = 1; i < size; i++)
-        src[i] += src[i-1] - 128;
+        src[i] = a;
+        src[i + 1] += a;
+        src[i] ^= 0x80;
+    }
 }
 
 av_cold void ff_exrdsp_init(ExrDSPContext *c)