0030782: Visualization, Font_FTFont - use predefined fallback fonts for extended Unicode subsets [backported to occt720]

Font_FTFont now uses fallback fonts for characters from unsupported Unicode subsets,
managed by Font_FTFont::ToUseUnicodeSubsetFallback()
and Font_FontMgr::ToUseUnicodeSubsetFallback() option, enabled by default.
The fallback list includes common font families for Chinese, Korean and Japanese languages.

Font_FTFont::RenderGlyph() now supports FT_PIXEL_MODE_MONO input format used by some CJK fonts.
OpenGl_Font::createTexture() now limits single texture size to circa 4096 glyphs.

test/testgrid now expects test scripts being in UTF-8 encoding in sync with "DRAWEXE -f script.tcl".

AIS::InitFaceLength() - fixed usage of uninitialized result.

Font_FTFont::RenderGlyph() backport bitmap

src/Font/Font_UnicodeSubset.hxx

@@ -0,0 +1,28 @@
+// Copyright (c) 2019 OPEN CASCADE SAS
+//
+// This file is part of Open CASCADE Technology software library.
+//
+// This library is free software; you can redistribute it and/or modify it under
+// the terms of the GNU Lesser General Public License version 2.1 as published
+// by the Free Software Foundation, with special exception defined in the file
+// OCCT_LGPL_EXCEPTION.txt. Consult the file LICENSE_LGPL_21.txt included in OCCT
+// distribution for complete text of the license and disclaimer of any warranty.
+//
+// Alternatively, this file may be used under the terms of Open CASCADE
+// commercial license or contractual agreement.
+
+#ifndef _Font_UnicodeSubset_HeaderFile
+#define _Font_UnicodeSubset_HeaderFile
+
+//! Enumeration defining Unicode subsets.
+enum Font_UnicodeSubset
+{
+  Font_UnicodeSubset_Western, //!< western letters
+  Font_UnicodeSubset_Korean,  //!< modern Korean letters
+  Font_UnicodeSubset_CJK,     //!< Chinese characters (Chinese, Japanese, Korean and Vietnam)
+  Font_UnicodeSubset_Arabic,  //!< Arabic  characters
+};
+
+enum { Font_UnicodeSubset_NB = Font_UnicodeSubset_Arabic };
+
+#endif // _Font_UnicodeSubset_HeaderFile