Let userland retain the window backing store while drawing into it.

To start painting, call:
gui$get_window_backing_store()

Then finish up with:
gui$release_window_backing_store()

Process will retain the underlying GraphicsBitmap behind the scenes.
This fixes racing between the WindowServer and GUI clients.

This patch also adds a WSWindowLocker that is exactly what it sounds like.

SharedGraphics/GraphicsBitmap.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "Color.h"
+#include "Rect.h"
 #include "Size.h"
 #include <AK/Retainable.h>
 #include <AK/RetainPtr.h>
@@ -20,6 +21,7 @@ public:
     RGBA32* scanline(int y);
     const RGBA32* scanline(int y) const;
 
+    Rect rect() const { return { {}, m_size }; }
     Size size() const { return m_size; }
     int width() const { return m_size.width(); }
     int height() const { return m_size.height(); }

SharedGraphics/Painter.cpp

@@ -7,6 +7,8 @@
 #ifdef LIBGUI
 #include <LibGUI/GWidget.h>
 #include <LibGUI/GWindow.h>
+#include <LibC/gui.h>
+#include <LibC/stdio.h>
 #endif
 
 #define DEBUG_WIDGET_UNDERDRAW
@@ -22,12 +24,20 @@ Painter::Painter(GraphicsBitmap& bitmap)
 Painter::Painter(GWidget& widget)
     : m_font(&widget.font())
 {
-    m_target = widget.backing();
+    GUI_WindowBackingStoreInfo backing;
+    int rc = gui_get_window_backing_store(widget.window()->window_id(), &backing);
+    if (rc < 0) {
+        perror("gui_get_window_backing_store");
+        exit(1);
+    }
+    m_backing_store_id = backing.backing_store_id;
+    m_target = GraphicsBitmap::create_wrapper(backing.size, backing.pixels);
     ASSERT(m_target);
     m_window = widget.window();
     m_translation.move_by(widget.relative_position());
     // NOTE: m_clip_rect is in Window coordinates since we are painting into its backing store.
     m_clip_rect = widget.relative_rect();
+    m_clip_rect.intersect(m_target->rect());
 
 #ifdef DEBUG_WIDGET_UNDERDRAW
     // If the widget is not opaque, let's not mess it up with debugging color.
@@ -39,6 +49,11 @@ Painter::Painter(GWidget& widget)
 
 Painter::~Painter()
 {
+#ifdef LIBGUI
+    m_target = nullptr;
+    int rc = gui_release_window_backing_store(m_backing_store_id);
+    ASSERT(rc == 0);
+#endif
 }
 
 void Painter::fill_rect(const Rect& a_rect, Color color)
@@ -47,6 +62,9 @@ void Painter::fill_rect(const Rect& a_rect, Color color)
     rect.move_by(m_translation);
     rect.intersect(m_clip_rect);
 
+    if (rect.is_empty())
+        return;
+
     RGBA32* dst = m_target->scanline(rect.top()) + rect.left();
     const unsigned dst_skip = m_target->width();
 
@@ -266,3 +284,13 @@ void Painter::blit(const Point& position, const GraphicsBitmap& source, const Re
         src += src_skip;
     }
 }
+
+void Painter::set_clip_rect(const Rect& rect)
+{
+    m_clip_rect = Rect::intersection(rect, m_target->rect());
+}
+
+void Painter::clear_clip_rect()
+{
+    m_clip_rect = m_target->rect();
+}

SharedGraphics/Painter.h

@@ -40,8 +40,8 @@ public:
     void set_draw_op(DrawOp op) { m_draw_op = op; }
     DrawOp draw_op() const { return m_draw_op; }
 
-    void set_clip_rect(const Rect& rect) { m_clip_rect = rect; }
-    void clear_clip_rect() { m_clip_rect = { 0, 0, 1024, 768 }; }
+    void set_clip_rect(const Rect& rect);
+    void clear_clip_rect();
     Rect clip_rect() const { return m_clip_rect; }
 
 private:
@@ -53,6 +53,7 @@ private:
     RetainPtr<GraphicsBitmap> m_target;
 #ifdef LIBGUI
     GWindow* m_window { nullptr };
+    void* m_backing_store_id { nullptr };
 #endif
     DrawOp m_draw_op { DrawOp::Copy };
 };