Fix array in matrixconvolution shader

TBR=halcanary@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1756353002

Review URL: https://codereview.chromium.org/1756353002
diff --git a/src/gpu/effects/GrMatrixConvolutionEffect.cpp b/src/gpu/effects/GrMatrixConvolutionEffect.cpp
index ea3301f..1dd96a4 100644
--- a/src/gpu/effects/GrMatrixConvolutionEffect.cpp
+++ b/src/gpu/effects/GrMatrixConvolutionEffect.cpp
@@ -39,8 +39,8 @@
     int kWidth = mce.kernelSize().width();
     int kHeight = mce.kernelSize().height();
 
-    int arrayCount = (kWidth + 3) / 4;
-    SkASSERT(4 * arrayCount >= kWidth);
+    int arrayCount = (kWidth * kHeight + 3) / 4;
+    SkASSERT(4 * arrayCount >= kWidth * kHeight);
 
     GrGLSLUniformHandler* uniformHandler = args.fUniformHandler;
     fImageIncrementUni = uniformHandler->addUniform(kFragment_GrShaderFlag,