Teach //examples/shadows to draw a soft shadow

After this CL, this example actually draws a soft shadow. There are still a
number of shortcuts that we'll need to unwind in order to produce a fully
working system, but this CL is at least a start.

R=jeffbrown@google.com, kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1810793002 .
diff --git a/examples/shadows/BUILD.gn b/examples/shadows/BUILD.gn
index f5477da..8e40db4 100644
--- a/examples/shadows/BUILD.gn
+++ b/examples/shadows/BUILD.gn
@@ -9,6 +9,8 @@
 
   sources = [
     "main.cc",
+    "penumbra_program.cc",
+    "penumbra_program.h",
     "shadows_app.cc",
     "shadows_app.h",
     "shadows_renderer.cc",