Implement 'leading' font metric for linux platform

Fixes #76.

Implemented the only font metric remaining to resolve issue #76 for the linux platform.

There are still some variations between linux and osx font metrics.  Looking at values for the "Times New Roman" font, the following metrics differed by over 10%:  `leading`, `x_height` and `max_advance`.  It was not obvious if this would create any major difference in rendering, or whether the discrepancies would be handled in platform specific ways during the rendering process.  Only `x_height` is being used by existing code.

#### Platform comparison of font metrics for "Times New Roman" between linux and OS X:
**Au** is the raw metric values stored in the `FontMetrics` struct.  **% of em** is how large that value is relative to the raw `em_size` of the font as a percentage.  Linux reports the pt size differently than OS X, but this is probably because the linux platform assumes the dpi is 72 (hard-coded `src/components/util/geometry.rs`), whereas OS X uses the dpi of the display.

    +-----------------------------------+  +-----------------------------------+
    | Linux (@14.25pt, 1.33em)          |  | OS X (@19.35pt, 1.0em)            |
    +------------------+------+---------+  +------------------+------+---------+
    | property         |   Au | % of em |  | property         |   Au | % of em |
    +------------------+------+---------+  +------------------+------+---------+
    | underline_size   |   55 |    4.82 |  | underline_size   |   60 |    5.17 |
    | underline_offset | -151 |  -13.25 |  | underline_offset | -120 |  -10.34 |
    | strikeout_size   |   56 |    4.91 |  | strikeout_size   |    0 |    0.00 |
    | strikeout_offset |  295 |   25.88 |  | strikeout_offset |    0 |    0.00 |
    | leading          |  541 |   47.45 |  | leading          |   60 |    5.17 |
    | x_height         | 1015 |   89.04 |  | x_height         |  660 |   56.85 |
    | em_size          | 1140 |  100.00 |  | em_size          | 1161 |  100.00 |
    | ascent           | 1015 |   89.04 |  | ascent           |  893 |   76.92 |
    | descent          |  246 |   21.58 |  | descent          |  203 |   17.48 |
    | max_advance      | 1199 |  105.18 |  | max_advance      | 2820 |  242.89 |
    +------------------+------+---------+  +------------------+------+---------+

    +-----------------------------------+  +-----------------------------------+
    | Linux (@24pt, 1.33em)             |  | OS X (@32.683333pt, 1.0em)        |
    +------------------+------+---------+  +------------------+------+---------+
    | property         |   Au | % of em |  | property         |   Au | % of em |
    +------------------+------+---------+  +------------------+------+---------+
    | underline_size   |   93 |    4.84 |  | underline_size   |  120 |    6.12 |
    | underline_offset | -255 |  -13.28 |  | underline_offset | -240 |  -12.24 |
    | strikeout_size   |   95 |    4.95 |  | strikeout_size   |    0 |    0.00 |
    | strikeout_offset |  496 |   25.83 |  | strikeout_offset |    0 |    0.00 |
    | leading          |  912 |   47.50 |  | leading          |   60 |    3.06 |
    | x_height         | 1710 |   89.06 |  | x_height         | 1080 |   55.10 |
    | em_size          | 1920 |  100.00 |  | em_size          | 1960 |  100.00 |
    | ascent           | 1710 |   89.06 |  | ascent           | 1544 |   78.77 |
    | descent          |  415 |   21.61 |  | descent          |  365 |   18.62 |
    | max_advance      | 2020 |  105.21 |  | max_advance      | 4740 |  241.84 |
    +------------------+------+---------+  +------------------+------+---------+

    +-----------------------------------+  +-----------------------------------+
    | Linux (@45pt, 1.33em)             |  | OS X (@60.833333pt, 1.0em)        |
    +------------------+------+---------+  +------------------+------+---------+
    | property         |   Au | % of em |  | property         |   Au | % of em |
    +------------------+------+---------+  +------------------+------+---------+
    | underline_size   |  175 |    4.86 |  | underline_size   |  180 |    4.93 |
    | underline_offset | -479 |  -13.31 |  | underline_offset | -480 |  -13.15 |
    | strikeout_size   |  179 |    4.97 |  | strikeout_size   |    0 |    0.00 |
    | strikeout_offset |  931 |   25.86 |  | strikeout_offset |    0 |    0.00 |
    | leading          | 1709 |   47.47 |  | leading          |  180 |    4.93 |
    | x_height         | 3208 |   89.11 |  | x_height         | 2040 |   55.89 |
    | em_size          | 3600 |  100.00 |  | em_size          | 3650 |  100.00 |
    | ascent           | 3208 |   89.11 |  | ascent           | 2925 |   80.14 |
    | descent          |  778 |   21.61 |  | descent          |  690 |   18.90 |
    | max_advance      | 3788 |  105.22 |  | max_advance      | 8880 |  243.29 |
    +------------------+------+---------+  +------------------+------+---------+

    +-----------------------------------+  +-----------------------------------+
    | Linux (@30.75pt, 1.33em)          |  | OS X (@41.9pt, 1.0em)             |
    +------------------+------+---------+  +------------------+------+---------+
    | property         |   Au | % of em |  | property         |   Au | % of em |
    +------------------+------+---------+  +------------------+------+---------+
    | underline_size   |  120 |    4.88 |  | underline_size   |  120 |    4.77 |
    | underline_offset | -327 |  -13.29 |  | underline_offset | -360 |  -14.32 |
    | strikeout_size   |  122 |    4.95 |  | strikeout_size   |    0 |    0.00 |
    | strikeout_offset |  636 |   25.85 |  | strikeout_offset |    0 |    0.00 |
    | leading          | 1168 |   47.47 |  | leading          |  120 |    4.77 |
    | x_height         | 2192 |   89.11 |  | x_height         | 1440 |   57.27 |
    | em_size          | 2460 |  100.00 |  | em_size          | 2514 |  100.00 |
    | ascent           | 2192 |   89.11 |  | ascent           | 1991 |   79.20 |
    | descent          |  532 |   21.63 |  | descent          |  487 |   19.37 |
    | max_advance      | 2588 |  105.20 |  | max_advance      | 6120 |  243.44 |
    +------------------+------+---------+  +------------------+------+---------+