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.

glslang/MachineIndependent/preprocessor/PpContext.cpp

@@ -83,8 +83,8 @@ NVIDIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 namespace glslang {
 
-TPpContext::TPpContext(TParseContext& pc) : 
-    preamble(0), strings(0), parseContext(pc), inComment(false)
+TPpContext::TPpContext(TParseContext& pc, const TShader::Includer& inclr) : 
+    preamble(0), strings(0), parseContext(pc), includer(inclr), inComment(false)
 {
     InitAtomTable();
     InitScanner();