Add support for GL_ARB_shading_language_include

Add support for GL_ARB_shading_language_include. Usage is identical to
the way GL_GOOGLE_include_directive currently works glslang (since
GL_ARB_shading_language_include is inherently a runtime feature and
glslang is an offline compiler).

Users can simulate their runtime environment by using a custom
glslang::TShader::Includer or using filenames that match their GL
runtime names.

Closes #249.

gtests/Pp.FromFile.cpp

@@ -67,6 +67,8 @@ INSTANTIATE_TEST_SUITE_P(
         "preprocessor.defined.vert",
         "preprocessor.many.endif.vert",
         "preprocessor.eof_missing.vert",
+        "preprocess.arb_shading_language_include.vert",
+        "preprocess.include_directive_missing_extension.vert",
     })),
     FileNameAsCustomTestSuffix
 );