Add option to control maximum GrBatch lookback

src/gpu/GrDrawTarget.cpp

Index: src/gpu/GrDrawTarget.cpp
diff --git a/src/gpu/GrDrawTarget.cpp b/src/gpu/GrDrawTarget.cpp
index 59488c4d0352b596ea32334b956a291955bdeb5e..55f66249847c1970733415521fe2ba05251e0347 100644
--- a/src/gpu/GrDrawTarget.cpp
+++ b/src/gpu/GrDrawTarget.cpp
@@ -32,6 +32,9 @@
 
 ////////////////////////////////////////////////////////////////////////////////
 
+// Experimentally we have found that most batching occurs within the first 10 comparisons.
+static const int kDefaultMaxBatchLookback = 10;
+
 GrDrawTarget::GrDrawTarget(GrRenderTarget* rt, GrGpu* gpu, GrResourceProvider* resourceProvider,
                            const Options& options)
     : fGpu(SkRef(gpu))
@@ -44,6 +47,8 @@ GrDrawTarget::GrDrawTarget(GrRenderTarget* rt, GrGpu* gpu, GrResourceProvider* r
     fClipMaskManager.reset(new GrClipMaskManager(this, options.fClipBatchToBounds));
 
     fDrawBatchBounds = options.fDrawBatchBounds;
+    fMaxBatchLookback = (options.fMaxBatchLookback < 0) ? kDefaultMaxBatchLookback :
+                                                          options.fMaxBatchLookback;
 
     rt->setLastDrawTarget(this);
 
@@ -469,8 +474,6 @@ void GrDrawTarget::recordBatch(GrBatch* batch) {
     // 1) check every draw
     // 2) intersect with something
     // 3) find a 'blocker'
-    // Experimentally we have found that most batching occurs within the first 10 comparisons.
-    static const int kMaxLookback = 10;
 
     GrBATCH_INFO("Re-Recording (%s, B%u)\n"
         "\tBounds LRTB (%f, %f, %f, %f)\n",
@@ -480,7 +483,7 @@ void GrDrawTarget::recordBatch(GrBatch* batch) {
         batch->bounds().fTop, batch->bounds().fBottom);
     GrBATCH_INFO(SkTabString(batch->dumpInfo(), 1).c_str());
     GrBATCH_INFO("\tOutcome:\n");    
-    int maxCandidates = SkTMin(kMaxLookback, fBatches.count());
+    int maxCandidates = SkTMin(fMaxBatchLookback, fBatches.count());
     if (maxCandidates) {
         int i = 0;
         while (true) {