dm: simplify parallel/serial decisions

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 3a7d4ee516cfa7ccb87e9a0e9c7cfbe3c7d19634..64897d20bcc7c04400d1eb54bc0397c2a8096a38 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -59,13 +59,15 @@ struct SinkFlags {
 };
 
 struct Src {
-    // All Srcs must be thread safe.
     virtual ~Src() {}
     virtual Error SK_WARN_UNUSED_RESULT draw(SkCanvas*) const = 0;
     virtual SkISize size() const = 0;
     virtual Name name() const = 0;
     virtual void modifyGrContextOptions(GrContextOptions* options) const {}
     virtual bool veto(SinkFlags) const { return false; }
+
+    // Force Tasks using this Src to run on the main thread?
+    virtual bool serial() const { return false; }
 };
 
 struct Sink {
@@ -73,8 +75,9 @@ struct Sink {
     // You may write to either the bitmap or stream.  If you write to log, we'll print that out.
     virtual Error SK_WARN_UNUSED_RESULT draw(const Src&, SkBitmap*, SkWStream*, SkString* log)
         const = 0;
-    // Sinks in the same enclave (except kAnyThread_Enclave) will run serially on the same thread.
-    virtual int enclave() const = 0;
+
+    // Force Tasks using this Sink to run on the main thread?
+    virtual bool serial() const { return false; }
 
     // File extension for the content draw() outputs, e.g. "png", "pdf".
     virtual const char* fileExtension() const  = 0;
@@ -82,9 +85,6 @@ struct Sink {
     virtual SinkFlags flags() const = 0;
 };
 
-enum { kAnyThread_Enclave, kGPU_Enclave };
-static const int kNumEnclaves = kGPU_Enclave + 1;
-
 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
 
 class GMSrc : public Src {
@@ -200,7 +200,6 @@ public:
     NullSink() {}
 
     Error draw(const Src& src, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return ""; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kNull, SinkFlags::kDirect }; }
 };
@@ -212,7 +211,7 @@ public:
             int samples, bool diText, bool threaded);
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override;
+    bool serial() const override { return !fThreaded; }
     const char* fileExtension() const override { return "png"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kGPU, SinkFlags::kDirect }; }
 private:
@@ -228,7 +227,6 @@ public:
     PDFSink(const char* rasterizer);
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return "pdf"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kVector, SinkFlags::kDirect }; }
 private:
@@ -240,7 +238,6 @@ public:
     XPSSink();
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return "xps"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kVector, SinkFlags::kDirect }; }
 };
@@ -250,7 +247,6 @@ public:
     explicit RasterSink(SkColorType);
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return "png"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kRaster, SinkFlags::kDirect }; }
 private:
@@ -262,7 +258,6 @@ public:
     SKPSink();
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return "skp"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kVector, SinkFlags::kDirect }; }
 };
@@ -272,7 +267,6 @@ public:
     SVGSink();
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
-    int enclave() const override { return kAnyThread_Enclave; }
     const char* fileExtension() const override { return "svg"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kVector, SinkFlags::kDirect }; }
 };
@@ -284,7 +278,7 @@ class Via : public Sink {
 public:
     explicit Via(Sink* sink) : fSink(sink) {}
     const char* fileExtension() const override { return fSink->fileExtension(); }
-    int               enclave() const override { return fSink->enclave(); }
+    bool               serial() const override { return fSink->serial(); }
     SinkFlags flags() const override {
         SinkFlags flags = fSink->flags();
         flags.approach = SinkFlags::kIndirect;