Add GpFontHandler_FreeType2 to CMake

Also add stddef.h include to GpRenderedGlyphMetrics.h to make size_t
visible. Does MSVC provide size_t by default, or via stdint.h? This
prevents compilation and I can't see how this would have compiled on
Windows otherwise.

GpCommon/GpRenderedGlyphMetrics.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <stddef.h>
 #include <stdint.h>
 
 struct GpRenderedGlyphMetrics