Add a debug flag for CSS deprecations

It seems better not to spam everybody with this all the time.

gtk/gtkcssprovider.c

@@ -184,6 +184,8 @@ gtk_css_provider_parsing_error (GtkCssProvider  *provider,
     {
       char *s = gtk_css_section_to_string (section);
 
+      if (GTK_DEBUG_CHECK (CSS) ||
+          !g_error_matches (error, GTK_CSS_PARSER_WARNING, GTK_CSS_PARSER_WARNING_DEPRECATED))
         g_warning ("Theme parser %s: %s: %s",
                    error->domain == GTK_CSS_PARSER_WARNING ? "warning" : "error",
                    s,

gtk/gtkdebug.h

@@ -96,6 +96,7 @@ typedef enum {
   GTK_DEBUG_A11Y            = 1 << 17,
   GTK_DEBUG_ICONFALLBACK    = 1 << 18,
   GTK_DEBUG_INVERT_TEXT_DIR = 1 << 19,
+  GTK_DEBUG_CSS             = 1 << 20,
 } GtkDebugFlags;
 
 #define GTK_DEBUG_CHECK(type) G_UNLIKELY (gtk_get_debug_flags () & GTK_DEBUG_##type)

gtk/gtkmain.c

@@ -200,6 +200,7 @@ static const GdkDebugKey gtk_debug_keys[] = {
   { "accessibility", GTK_DEBUG_A11Y, "Information about accessibility state changes" },
   { "iconfallback", GTK_DEBUG_ICONFALLBACK, "Information about icon fallback" },
   { "invert-text-dir", GTK_DEBUG_INVERT_TEXT_DIR, "Invert the default text direction" },
+  { "css", GTK_DEBUG_CSS, "Information about deprecated CSS features" },
 };
 
 /* This checks to see if the process is running suid or sgid