Ligature support

Description

This patch adds proper drawing of ligatures.

The code uses Harfbuzz library to transform original text of a single line to a list of glyphs with ligatures included.

Taken with Fira Code Regular 14pt:

The patch adds additional dependency on Harfbuzz library and headers.
Original patch was made for vanilla version of ST from latest master commit. It is not 100% compatible with Scrollback and Alpha patches, so I made modified versions that you can apply on top of a Scrollback and/or Alpha patch.
Due to some limitations in drawing engine, ligatures will break when crossing colors, font styles or selection. They will still render properly as separate symbols, just not as ligatures.
Since 0.8.4 patch, there's now a way to enable additional font rendering features. Look into features array in hb.c for details.

The original patch does not work very well with the boxdraw patch. Since it requires some additional changes in the code to make ligatures compatible with boxdraw, a special version of the patch was added, that you can apply on top of the boxdraw patch.
It does not include Alpha or Scrollback patches.

0.9.2:

0.9:

0.8.4:

0.8.3:

0.8.2-28ad288: