In SkSLSlide, directly use Viewer's shader error handler

With the CPU backend, there is no GrContext on the canvas, so we were
sending errors to the default handler (SkDebugf + assert), so editing
shaders was impossible. Now they fail gracefully (and produce a popup
window with the message).

Change-Id: I29bad24f201be59ba1cec45f446a433c01cf86dc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297461
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/tools/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index fa7b4f7..973171a 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -79,6 +79,8 @@
 
 static CapturingShaderErrorHandler gShaderErrorHandler;
 
+GrContextOptions::ShaderErrorHandler* Viewer::ShaderErrorHandler() { return &gShaderErrorHandler; }
+
 using namespace sk_app;
 
 static std::map<GpuPathRenderers, std::string> gPathRendererNames;