Enable the ESLint operator-assignment rule

This patch was generated automatically, using the `gulp lint --fix` command.

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/operator-assignment

src/core/image.js

@@ -471,7 +471,7 @@ class PDFImage {
           value = max;
         }
         output[i] = value;
-        buf = buf & ((1 << remainingBits) - 1);
+        buf &= (1 << remainingBits) - 1;
         bits = remainingBits;
       }
     }