Introduce `FT_THROW' macro.

The idea is to replace code like

  return FT_Err_Foo_Bar;

or

  return CFF_Err_Foo_Bar;

with

  return FT_THROW( Foo_Bar );

The FT_THROW macro has two functions:

  . It hides the module specific prefix.

  . In debug mode, it calls the empty function `FT_Throw' which can
    be thus used to set a breakpoint.

* include/freetype/internal/ftdebug.h (FT_THROW): New macro.
(FT_Throw): New prototype.
* src/base/ftdebug.c (FT_Throw): New function.
diff --git a/src/base/ftdebug.c b/src/base/ftdebug.c
index 2adbeab..cada8d0 100644
--- a/src/base/ftdebug.c
+++ b/src/base/ftdebug.c
@@ -77,6 +77,21 @@
     exit( EXIT_FAILURE );
   }
 
+
+  /* documentation is in ftdebug.h */
+
+  FT_BASE_DEF( int )
+  FT_Throw( FT_Error     error,
+            int          line,
+            const char*  file )
+  {
+    FT_UNUSED( error );
+    FT_UNUSED( line );
+    FT_UNUSED( file );
+
+    return 0;
+  }
+
 #endif /* FT_DEBUG_LEVEL_ERROR */