checkasm/vf_blend: rename addition128 and difference128 to grainmerge and grainextract

This was missing from f8d0689d.
Fixes checkasm.

tests/checkasm/vf_blend.c

@@ -103,11 +103,11 @@ void checkasm_check_blend(void)
         check_blend_func();
 
     check_and_report(addition, BLEND_ADDITION)
-    check_and_report(addition128, BLEND_ADDITION128)
+    check_and_report(grainmerge, BLEND_GRAINMERGE)
     check_and_report(and, BLEND_AND)
     check_and_report(average, BLEND_AVERAGE)
     check_and_report(darken, BLEND_DARKEN)
-    check_and_report(difference128, BLEND_DIFFERENCE128)
+    check_and_report(grainextract, BLEND_GRAINEXTRACT)
     check_and_report(hardmix, BLEND_HARDMIX)
     check_and_report(lighten, BLEND_LIGHTEN)
     check_and_report(multiply, BLEND_MULTIPLY)