Fix high byte of I;16* images being truncated by ImageFilter

Follows up on #8438 (418ae7c)

Author: akx

Tests/test_image_filter.py

@@ -210,6 +210,16 @@ def test_consistency_5x5(mode: str) -> None:
             assert_image_equal(source.filter(kernel), reference)
 
 
+@pytest.mark.parametrize("mode", ("I;16", "I;16L", "I;16B", "I;16N"))
+def test_consistency_i16_high_byte(mode: str) -> None:
+    # Exercise filters with a 16bpc image that has content in the high byte, too.
+    im = Image.new(mode, (8, 8), 1000)
+    pixel = im.filter(ImageFilter.SMOOTH).getpixel((4, 4))
+    assert isinstance(pixel, (int, float))
+    # Allow for kernel rounding errors.
+    assert abs(pixel - 1000) <= 1
+
+
 @pytest.mark.parametrize(
     "radius",
     (

src/libImaging/Filter.c

@@ -114,7 +114,7 @@ kernel_i16(int size, UINT8 *in0, int x, const float *kernel, int bigendian) {
     for (i = 0; i < size; i++) {
         int x1 = x + i - half_size;
         result += (float)(in0[x1 * 2 + (bigendian ? 1 : 0)] +
-                          (in0[x1 * 2 + (bigendian ? 0 : 1)] >> 8)) *
+                          (in0[x1 * 2 + (bigendian ? 0 : 1)] << 8)) *
                   kernel[i];
     }
     return result;