enforce 16byte alignment in shader contexts

src/core/SkComposeShader.cpp

Index: src/core/SkComposeShader.cpp
diff --git a/src/core/SkComposeShader.cpp b/src/core/SkComposeShader.cpp
index 46ff4019fc22f323649699146f3ba8695f34c3f6..e4ad6e9edab9099197a5b268b9446e6759b48d14 100644
--- a/src/core/SkComposeShader.cpp
+++ b/src/core/SkComposeShader.cpp
@@ -30,8 +30,8 @@ SkComposeShader::~SkComposeShader() {
     fShaderA->unref();
 }
 
-size_t SkComposeShader::contextSize(const ContextRec& rec) const {
-    return sizeof(ComposeShaderContext)
+size_t SkComposeShader::onContextSize(const ContextRec& rec) const {
+    return SkAlign16(sizeof(ComposeShaderContext))
         + fShaderA->contextSize(rec)
         + fShaderB->contextSize(rec);
 }
@@ -76,7 +76,7 @@ template <typename T> void safe_call_destructor(T* obj) {
 }
 
 SkShader::Context* SkComposeShader::onCreateContext(const ContextRec& rec, void* storage) const {
-    char* aStorage = (char*) storage + sizeof(ComposeShaderContext);
+    char* aStorage = (char*) storage + SkAlign16(sizeof(ComposeShaderContext));
     char* bStorage = aStorage + fShaderA->contextSize(rec);
 
     // we preconcat our localMatrix (if any) with the device matrix