Change GaussianBlur logic wrt color spaces

I was using 'source' where I should have said 'input'. Also, to be
consistent with other image filters, ensure that the input is in the
destination gamut before we start blurring.

BUG=skia:

Change-Id: I751961b42a2a5d110ee8ea8916279c8fe0d5248e
Reviewed-on: https://skia-review.googlesource.com/6486
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/gm/imagefilters.cpp b/gm/imagefilters.cpp
index 27422b5..8855afd 100644
--- a/gm/imagefilters.cpp
+++ b/gm/imagefilters.cpp
@@ -68,7 +68,7 @@
 }
 
 static sk_sp<SkImage> make_image(SkCanvas* canvas) {
-    const SkImageInfo info = SkImageInfo::MakeN32Premul(100, 100);
+    const SkImageInfo info = SkImageInfo::MakeS32(100, 100, kPremul_SkAlphaType);
     auto surface(canvas->makeSurface(info));
     if (!surface) {
         surface = SkSurface::MakeRaster(info);