LibWeb: Implement screenshot painting inside Web::WebDriver

This will allow for implementing the screenshot action closer to the
spec, as we can now use HTMLCanvasElement to encode the bitmap, and
capture the screenshot on the animation frame loop.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -442,6 +442,7 @@ set(SOURCES
     WebDriver/Error.cpp
     WebDriver/ExecuteScript.cpp
     WebDriver/Response.cpp
+    WebDriver/Screenshot.cpp
     WebGL/WebGLContextAttributes.cpp
     WebGL/WebGLContextEvent.cpp
     WebGL/WebGLRenderingContext.cpp