Make GrOp::onPrePrepare be pure virtual

This makes onPrePrepare match onPrepare & onExecute. Most of the new method bodies will-have-to/should be filled in anyway.

Change-Id: Ifc897feaeb2d8fe6eec3ac3a8e91f99393ad6758
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277542
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/samplecode/SampleCCPRGeometry.cpp b/samplecode/SampleCCPRGeometry.cpp
index fb7f7a6..9b150e2 100644
--- a/samplecode/SampleCCPRGeometry.cpp
+++ b/samplecode/SampleCCPRGeometry.cpp
@@ -94,6 +94,10 @@
                                       bool hasMixedSampledCoverage, GrClampType) override {
         return GrProcessorSet::EmptySetAnalysis();
     }
+    void onPrePrepare(GrRecordingContext*,
+                      const GrSurfaceProxyView* outputView,
+                      GrAppliedClip*,
+                      const GrXferProcessor::DstProxyView&) override {}
     void onPrepare(GrOpFlushState*) override {}
     void onExecute(GrOpFlushState*, const SkRect& chainBounds) override;