Implement 'leading' font metric for linux platform

[dhedlund]

Fixes #76.

Implemented the only font metric remaining to resolve issue #76 for the Linux platform.

There are still some variations between Linux and OS X 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. The leading property under Linux is still an order of magnitude off from the OS X version, but they are computed with similar calculations. Issue #1355 has been opened to further investigate the discrepancies.

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 uses hard-coded points-per-inch value (hard-coded in src/components/util/geometry.rs), whereas OS X uses the points-per-inch value provided by the font library.

+-----------------------------------+  +-----------------------------------+
| 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 |
+------------------+------+---------+  +------------------+------+---------+

[metajack]

Awesome! Just had the one nit to clean up the history, otherwise r=me.

[dhedlund]

PR commit list should be cleaned up now.

[jdm]

r=metajack

[bors-servo]

saw approval from jdm, metajack
at dhedlund@79789a1

[bors-servo]

merging dhedlund/servo/issue_76 = 79789a1 into auto

[bors-servo]

dhedlund/servo/issue_76 = 79789a1 merged ok, testing candidate = 1bf28eb

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/690
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/645

[bors-servo]

fast-forwarding master to auto = 1bf28eb