debugger: SW rasterize the picture only when it changes

debugger: SW rasterize the picture only when it changes

Rasterize the picture only when it changes, not every time
raster widget paints itself.

Removes include SkForceLinking.h while organizing the SkDebuggerGUI.cpp
includes.

This is part of the work trying to remove bugs that come from
SkDebugCanvas stateful draw. Part of the state comes from some
optimization that this tries to reproduce in a different way.

Review URL: https://codereview.chromium.org/839743003