[db] fix design size scaling

As discussed in issue #398.

Ad futuram rei memoriam the gist of it:

- For the index, all values are scaled (decipoints * sp) / 10 *
  (7227 / 7200).

- The ``bp`` case (the default, OT-standard), needs no conversion
  because it matches how values are stored in the index.

- The ``pt`` case essentially reverts the bp→pt part of scaling
  done for the database by scaling the asked size by the same
  factor, i. e. by 7227 / 7200.

- The ``dd`` needs an extra 1238 / 1157.

Requesting a font at 10pt will then:

- ask for a size of 655360 for ``bp`` / default;
- ask for 657817 for ``pt``;
- ask for 703870 for ``dd``.

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