Import freetype-gl source

Retrieved from https://github.com/rougier/freetype-gl/
commit 469d9d26d2411e4915fb5e3451a9d3096a7d0047

Includes the patches that were previously applied at build time.

Also reverted commit 48e7e0f68bb4979b3884cf1b07886e7092c8366d -
Issue 62 (Google Code): use GL_RED instead of deprecated GL_ALPHA as
2D-texture's internalFormat
freetype-gl: remove unused files
freetype-gl: partial whitespace cleanup
freetype-gl: remove strndup (no longer used)
freetype-gl: remove platform.{c,h}

stdint.h and round() are available in new enough MSVC
freetype-gl: make texture_font_load_glyphs private

Since it is now only used to load one glyph at a time,
also rename it to texture_font_load_glyph and remove the ability to loop
over a string of wchar_t.
freetype-gl: use uint32_t instead of wchar_t

This allows representing the full set of Unicode characters
even on Win32, where wchar_t is only 16 bits.