[graphite] Handle dynamic instances that never accumulated anything

In the curve tessellating render step, a path that is only linear
segments is entirely handled by the inner triangles, so the patch
writing never actually emits any instances. This is detected after
the fact by seeing that the per-instance vertex count remains 0, so
we just skip issuing the draw call.

Change-Id: I708797484073f5afcfb5d734467378861091ce8a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/542616
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>

src/gpu/graphite/DrawWriter.cpp

@@ -83,7 +83,9 @@ void DrawWriter::setTemplate(BindBufferInfo vertices,
 }
 
 void DrawWriter::flush() {
-    if (fPendingCount == 0) {
+    // If nothing was appended, or the only appended data was through dynamic instances and the
+    // final vertex count per instance is 0 (-1 in the sign encoded field), nothing should be drawn.
+    if (fPendingCount == 0 || fTemplateCount == -1) {
         return;
     }
     if (fPendingBufferBinds) {

src/gpu/graphite/DrawWriter.h

@@ -343,6 +343,7 @@ public:
               BindBufferInfo indices,
               unsigned int vertexCount)
             : Appender(w, Target::kInstances) {
+        SkASSERT(vertexCount > 0);
         w.setTemplate(vertices, indices, w.fInstances, SkTo<int>(vertexCount));
     }
 
@@ -380,8 +381,7 @@ public:
 
 private:
     void updateTemplateCount() {
-        unsigned int count = static_cast<unsigned int>(fProxy);
-        SkASSERT(count > 0);
+        const unsigned int count = static_cast<unsigned int>(fProxy);
         fDrawer.fTemplateCount = std::min(fDrawer.fTemplateCount, -SkTo<int>(count) - 1);
         // By resetting the proxy after updating the template count, the next batch will start over
         // with the minimum required vertex count and grow from there.