Add option to control maximum GrBatch lookback

include/gpu/GrContextOptions.h

Index: include/gpu/GrContextOptions.h
diff --git a/include/gpu/GrContextOptions.h b/include/gpu/GrContextOptions.h
index 2874b52be7a64c9c929c0edd18d91474f1165833..8e6368a38960574dcede2bd88c084ff421c2025f 100644
--- a/include/gpu/GrContextOptions.h
+++ b/include/gpu/GrContextOptions.h
@@ -22,6 +22,7 @@ struct GrContextOptions {
         , fImmediateMode(false)
         , fClipBatchToBounds(false)
         , fDrawBatchBounds(false)
+        , fMaxBatchLookback(-1)
         , fUseShaderSwizzling(false) {}
 
     // EXPERIMENTAL
@@ -63,6 +64,9 @@ struct GrContextOptions {
         of their dev bounds. */
     bool fDrawBatchBounds;
 
+    /** For debugging, override the default maximum look-back window for GrBatch combining. */
+    int fMaxBatchLookback;
+
     /** Force us to do all swizzling manually in the shader and don't rely on extensions to do
         swizzling. */
     bool fUseShaderSwizzling;