Fix Noto font bug

A little bit of context.
While working with pdfkit we found this error.
foliojs/pdfkit#1106
We narrowed down the repro to fontkit with this little script
```
const fs = require('fs');
const notosans = require('fontkit').openSync('./NotoSansCJKsc-Bold.otf');
const subsetFor = (text, file) => {
  const run = notosans.layout(text);
  const subset = notosans.createSubset();
  run.glyphs.forEach((glyph) => subset.includeGlyph(glyph));
  subset.encodeStream().pipe(fs.createWriteStream(file));
};
subsetFor('间。楼', 'output.ttf');
```
With this script you can see that glyphs were missing on the subset font if you opened the font with something like fontforge

Regarding the logic behind this fix, when `!used_fds[fd]` was false, last items in `topDict.FDArray` and `used_subrs` weren't really the ones corresponding to the current `fd`. So we are saving a dictionary to find the real index were those values were stored.

What do you think about this fix?

src/subset/CFFSubset.js

@@ -55,6 +55,7 @@ export default class CFFSubset extends Subset {
 
     let used_fds = {};
     let used_subrs = [];
+    let fd_select = {};
     for (let gid of this.glyphs) {
       let fd = this.cff.fdForGlyph(gid);
       if (fd == null) {
@@ -64,15 +65,16 @@ export default class CFFSubset extends Subset {
       if (!used_fds[fd]) {
         topDict.FDArray.push(Object.assign({}, this.cff.topDict.FDArray[fd]));
         used_subrs.push({});
+        fd_select[fd] = topDict.FDArray.length - 1;
       }
 
       used_fds[fd] = true;
-      topDict.FDSelect.fds.push(topDict.FDArray.length - 1);
+      topDict.FDSelect.fds.push(fd_select[fd]);
 
       let glyph = this.font.getGlyph(gid);
       let path = glyph.path; // this causes the glyph to be parsed
       for (let subr in glyph._usedSubrs) {
-        used_subrs[used_subrs.length - 1][subr] = true;
+        used_subrs[fd_select[fd]][subr] = true;
       }
     }