Added missing ARB_shading_language_packing extension.

It contains a few (un)packing functions present in ARB_gpu_shader5, but
isn't mentioned anywhere except GLSL 4.20 changelog (and there it has
its old name, ARB_shading_language_pack2f). It might be supported on
machines where ARB_gpu_shader5 can't be supported (GL3 HW).

src/Context.cpp

@@ -174,6 +174,8 @@ const std::vector<Extension>& Extension::extensions(Version version) {
         _extension(GL,ARB,map_buffer_alignment),
         _extension(GL,ARB,shader_atomic_counters),
         _extension(GL,ARB,shader_image_load_store),
+        /* Mentioned in GLSL 4.20 specs as newly added */
+        _extension(GL,ARB,shading_language_packing),        // done
         _extension(GL,ARB,texture_storage)};
     static const std::vector<Extension> extensions430{
         _extension(GL,ARB,arrays_of_arrays),                // done

src/Extensions.h

@@ -131,6 +131,7 @@ namespace GL {
         _extension(GL,ARB,map_buffer_alignment,         GL210, GL420) // #113
         _extension(GL,ARB,shader_atomic_counters,       GL300, GL420) // #114
         _extension(GL,ARB,shader_image_load_store,      GL300, GL420) // #115
+        _extension(GL,ARB,shading_language_packing, /*?*/ GL210, GL420) // #116
         _extension(GL,ARB,texture_storage,              GL210, GL420) // #117
         _extension(GL,ARB,arrays_of_arrays,             GL210, GL430) // #120
         _extension(GL,ARB,clear_buffer_object,          GL210, GL430) // #121