Add mixed samples support to XPs

src/gpu/effects/GrCoverageSetOpXP.cpp

Index: src/gpu/effects/GrCoverageSetOpXP.cpp
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index 0023752a87cc519bd25710f925f9c8ecca1f2ed1..c732098679e53d3350221cb6f6707ef17cfff35d 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -224,7 +224,16 @@ GrXferProcessor*
 GrCoverageSetOpXPFactory::onCreateXferProcessor(const GrCaps& caps,
                                                 const GrProcOptInfo& colorPOI,
                                                 const GrProcOptInfo& covPOI,
+                                                bool hasMixedSamples,
                                                 const DstTexture* dst) const {
+    // We don't support inverting coverage with mixed samples. We don't expect to ever want this in
+    // the future, however we could at some point make this work using an inverted coverage
+    // modulation table. Note that an inverted table still won't work if there are coverage procs.
+    if (fInvertCoverage && hasMixedSamples) {
+        SkASSERT(false);
+        return NULL;
+    }
+
     return CoverageSetOpXP::Create(fRegionOp, fInvertCoverage);
 }