LibAccelGfx+LibWeb: Add support for stacking context opacity

For each stacking context with an opacity less than 1, we create a
separate framebuffer. We then blit the texture attached to this
framebuffer with the specified opacity.

To avoid the performance overhead of reading pixels from the texture
into Gfx::Bitmap, a new method that allows for direct blitting from
the texture is introduced, named blit_scaled_texture().

Userland/Services/WebContent/PageHost.h

@@ -143,10 +143,6 @@ private:
     Web::CSS::PreferredColorScheme m_preferred_color_scheme { Web::CSS::PreferredColorScheme::Auto };
 
     RefPtr<WebDriverConnection> m_webdriver;
-
-#ifdef HAS_ACCELERATED_GRAPHICS
-    OwnPtr<AccelGfx::Painter> m_accelerated_painter;
-#endif
 };
 
 }