don't draw non-finite contexts in imagefilters

Bug: skia:7507
Change-Id: Ifc210951c17f74770f15de1c2e13b42117a3354e
Reviewed-on: https://skia-review.googlesource.com/101202
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

include/core/SkImageFilter.h

@@ -66,6 +66,16 @@ public:
         SkImageFilterCache* cache() const { return fCache; }
         const OutputProperties& outputProperties() const { return fOutputProperties; }
 
+        /**
+         *  Since a context can be build directly, its constructor has no chance to
+         *  "return null" if it's given invalid or unsupported inputs. Call this to
+         *  know of the the context can be used.
+         *
+         *  The SkImageFilterCache Key, for example, requires a finite ctm (no infinities
+         *  or NaN), so that test is part of isValid.
+         */
+        bool isValid() const { return fCTM.isFinite(); }
+
     private:
         SkMatrix               fCTM;
         SkIRect                fClipBounds;

src/core/SkImageFilter.cpp

@@ -197,6 +197,9 @@ void SkImageFilter::flatten(SkWriteBuffer& buffer) const {
 sk_sp<SkSpecialImage> SkImageFilter::filterImage(SkSpecialImage* src, const Context& context,
                                                  SkIPoint* offset) const {
     SkASSERT(src && offset);
+    if (!context.isValid()) {
+        return nullptr;
+    }
 
     uint32_t srcGenID = fUsesSrcInput ? src->uniqueID() : 0;
     const SkIRect srcSubset = fUsesSrcInput ? src->subset() : SkIRect::MakeWH(0, 0);

src/core/SkImageFilterCache.h

@@ -28,6 +28,7 @@ struct SkImageFilterCacheKey {
                                      sizeof(SkIRect) + sizeof(uint32_t) + 4 * sizeof(int32_t),
                                      "image_filter_key_tight_packing");
         fMatrix.getType();  // force initialization of type, so hashes match
+        SkASSERT(fMatrix.isFinite());   // otherwise we can't rely on == self when comparing keys
     }
 
     uint32_t fUniqueID;