Correct greyscale PNG scaling factor. Non-interlaced greyscale PNG tests now pass.

basis/images/png/png-tests.factor

@@ -7,6 +7,11 @@ IN: images.png.tests
 
 ! The subset of the suite that should work given the current implementation.
 "vocab:images/testing/png" [
+    "basn0g01.png" decode-test
+    "basn0g02.png" decode-test
+    "basn0g04.png" decode-test
+    "basn0g08.png" decode-test
+    "basn0g16.png" decode-test
     "basn2c08.png" decode-test
     "basn6a08.png" decode-test
     "f00n2c08.png" decode-test

basis/images/png/png.factor

@@ -240,7 +240,7 @@ ERROR: unknown-component-type n ;
 : scale-factor ( n -- n' )
     {
         { 1 [ 255 ] }
-        { 2 [ 127 ] }
+        { 2 [ 85 ] }
         { 4 [ 17 ] }
     } case ;