glsl: GLSL extensions have the GL_ prefix

Both the #extension directive name and the preprocessor symbol start
with the GL_ prefix.  For example:

...

New glean/glsl1 tests have been added to test the #extension feature.

src/mesa/shader/slang/slang_compile.c

@@ -2608,8 +2608,8 @@ compile_with_grammar(const char *source,
       return GL_FALSE;
    }
 
-   if (sl_pp_context_add_extension(context, "ARB_draw_buffers", "GL_ARB_draw_buffers") ||
-       sl_pp_context_add_extension(context, "ARB_texture_rectangle", "GL_ARB_texture_rectangle")) {
+   if (sl_pp_context_add_extension(context, "GL_ARB_draw_buffers", "GL_ARB_draw_buffers") ||
+       sl_pp_context_add_extension(context, "GL_ARB_texture_rectangle", "GL_ARB_texture_rectangle")) {
       slang_info_log_error(infolog, "%s", sl_pp_context_error_message(context));
       sl_pp_context_destroy(context);
       return GL_FALSE;