Outline tags are chars not i8s

[asajeffrey]

In font-renderer/src/freetype/outline.rs there's an assumption that c_char is i8, so it fails to compile on architectures where c_char is u8:

info: component 'rust-std' for target 'aarch64-linux-android' is up to date
   Compiling pathfinder_font_renderer v0.5.0 (/Users/ajeffrey/github/asajeffrey/pathfinder/font-renderer)
   Compiling script_layout_interface v0.0.1 (/Users/ajeffrey/github/asajeffrey/servo/components/script_layout_interface)
   Compiling layout_traits v0.0.1 (/Users/ajeffrey/github/asajeffrey/servo/components/layout_traits)
error[E0308]: mismatched types
  --> /Users/ajeffrey/github/asajeffrey/pathfinder/font-renderer/src/freetype/outline.rs:65:24
   |
65 |             (position, tag)
   |                        ^^^ expected i8, found u8

error: aborting due to previous error