Add #include processing to glslang (though turned off by default).

When an include directive is recognized by the preprocessor, it
executes a callback on the filepath argument to obtain the file
contents.  That way the compilation client can deal with the file
system, include paths, etc.

Currently only accepts quoted filepaths -- no angle brackets yet.

Test/preprocessor.include.vert

@@ -0,0 +1,6 @@
+#line 8000
+#include
+#include 123
+#include "foo"
+#include "foo" garbage
+#include "no-eol"