bitmaptools: dither: get rid of shifts

this happens to make the occasional FS dither artifact disappear.
I guess `a * b >> 8` and `(a * b) / 256` are not identical.  I'm not
sure if it was just the parens or not, but write the clearer code and
rely on the compiler to substitute an appropriate shift if possible.

Author: jepler

shared-module/bitmaptools/__init__.c

@@ -749,9 +749,9 @@ void common_hal_bitmaptools_dither(displayio_bitmap_t *dest_bitmap, displayio_bi
                 int x1 = x + info->terms[i].dx;
                 int dy = info->terms[i].dy;
 
-                rows[dy][x1] = ((info->terms[i].dl * err) >> 8) + rows[dy][x1];
+                rows[dy][x1] = ((info->terms[i].dl * err) / 256) + rows[dy][x1];
             }
-            err = err * info->dl >> 8;
+            err = (err * info->dl) / 256;
         }
         write_pixels(dest_bitmap, y, out);
 
@@ -779,9 +779,9 @@ void common_hal_bitmaptools_dither(displayio_bitmap_t *dest_bitmap, displayio_bi
                 int x1 = x - info->terms[i].dx;
                 int dy = info->terms[i].dy;
 
-                rows[dy][x1] = ((info->terms[i].dl * err) >> 8) + rows[dy][x1];
+                rows[dy][x1] = ((info->terms[i].dl * err) / 256) + rows[dy][x1];
             }
-            err = err * info->dl >> 8;
+            err = (err * info->dl) / 256;
         }
         write_pixels(dest_bitmap, y, out);