Don't flush on read/write pixels unless necessary

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index eb8455adbbae89355d4521fbbbeb140ede4643f7..afe396d6536f8c02c3a08f83923f559846213075 100755
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -1347,7 +1347,7 @@ bool GrContext::writeTexturePixels(GrTexture* texture,
         }
     }
 
-    if (!(kDontFlush_PixelOpsFlag & flags)) {
+    if (!(kDontFlush_PixelOpsFlag & flags) && texture->hasPendingIO()) {
         this->flush();
     }
 
@@ -1418,7 +1418,7 @@ bool GrContext::readRenderTargetPixels(GrRenderTarget* target,
         }
     }
 
-    if (!(kDontFlush_PixelOpsFlag & flags)) {
+    if (!(kDontFlush_PixelOpsFlag & flags) && target->hasPendingWrite()) {
         this->flush();
     }