Enable the ESLint prefer-const rule globally (PR 11450 follow-up)

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

With the recent introduction of Prettier this sort of mass enabling of ESLint rules becomes a lot easier, since the code will be automatically reformatted as necessary to account for e.g. changed line lengths.

Note that this patch is generated automatically, by using the ESLint `--fix` argument, and will thus require some additional clean-up (which is done separately).

src/core/jpx.js

@@ -1541,7 +1541,7 @@ var JpxImage = (function JpxImageClosure() {
             y0 = y0items[j] + offset;
             y1 = y1items[j];
             y2 = y2items[j];
-            let g = y0 - ((y2 + y1) >> 2);
+            const g = y0 - ((y2 + y1) >> 2);
 
             out[pos++] = (g + y2) >> shift;
             out[pos++] = g >> shift;