Use standard inclusion scheme for FreeType headers. (issue 35580043)

[dak]

https://codereview.appspot.com/35580043/diff/1/lily/open-type-font.cc
File lily/open-type-font.cc (right):

https://codereview.appspot.com/35580043/diff/1/lily/open-type-font.cc#newcode26
lily/open-type-font.cc:26: #include FT_TRUETYPE_TABLES_H
This is inscrutable programming.  It relies on open-type-font.hh
including font-metric.hh which in turn includes freetype.hh which
happens to include the library header ft2build.h defining macros like
FT_TRUETYPE_TABLES_H.

Please, if you use identifiers defined in a particular header file,
include that particular header file instead of assuming that if it
magically seems to be defined for some reason, everything will turn out
fine.  The compile time difference is minuscule if a header file turns
out to get included more than once.

https://codereview.appspot.com/35580043/diff/1/lily/pango-font.cc
File lily/pango-font.cc (right):

https://codereview.appspot.com/35580043/diff/1/lily/pango-font.cc#newcode25
lily/pango-font.cc:25: #include FT_XFREE86_H
This is even more reckless programming.  It relies on pango/pangoft2.h
including the particular header file from freetype that defines
FT_XFREE86_H.

This issue is called "Use standard inclusion scheme for FreeType
headers", and the "standard inclusion scheme" will most definitely not
state that Freetype headers will be incidentally defined by loading some
pango library.

That's not just inscrutable, it is completely out of our control.

https://codereview.appspot.com/35580043/