Add GM to exercise some of the darker corners of SkMagnifierImageFilter

An upcoming CL (https://skia-review.googlesource.com/c/7995/ (Remove asTextureRef from SkSpecialImage & update effects accordingly)) modifies some untested portions of the SkMagnifierImageFilter.

This adds a test to prevents regressions.

Change-Id: I9fa406f699e39fa393212e7f63a457b015b36edb
Reviewed-on: https://skia-review.googlesource.com/8023
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index 409376c..b1d3c4ad 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -253,6 +253,7 @@
   "$_gm/shapes.cpp",
   "$_gm/showmiplevels.cpp",
   "$_gm/simpleaaclip.cpp",
+  "$_gm/simple_magnification.cpp",
   "$_gm/simplerect.cpp",
   "$_gm/skbug_257.cpp",
   "$_gm/skbug_4868.cpp",