Build Painter & friends into LibC. Use it in the GUI test app.

Kernel/ProcessGUI.cpp

@@ -111,6 +111,8 @@ int Process::gui$invalidate_window(int window_id)
     if (it == m_windows.end())
         return -EBADWINDOW;
     auto& window = *(*it).value;
+    // FIXME: This should queue up a message that the window server process can read.
+    //        Poking into its data structures is not good.
     WindowManager::the().invalidate(window);
     return 0;
 }

Kernel/kprintf.cpp

@@ -39,7 +39,7 @@ static void debugger_putch(char*&, char ch)
     IO::out8(0xe9, ch);
 }
 
-int dbgprintf(const char* fmt, ...)
+extern "C" int dbgprintf(const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);

Kernel/kprintf.h

@@ -2,7 +2,8 @@
 
 #include <AK/Compiler.h>
 
+extern "C" {
 int dbgprintf(const char *fmt, ...);
 int kprintf(const char *fmt, ...);
 int ksprintf(char* buf, const char *fmt, ...);
-
+}