LibGfx: Make all fill_path() code member functions and move into .cpp

This makes all the code for fill_path() member functions of the painter,
and moves them into a new FillPathImplementation.cpp. This allows us
to avoid polluting Painter.h with implementation details, and makes
the edit, compile, retry loop much shorter.

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -10,6 +10,7 @@ set(SOURCES
     Color.cpp
     CursorParams.cpp
     DDSLoader.cpp
+    FillPathImplementation.cpp
     Filters/ColorBlindnessFilter.cpp
     Filters/FastBoxBlurFilter.cpp
     Filters/LumaFilter.cpp