softgpu: Avoid unnecessary flushing for curves.

We don't need to flush all drawing between curves in softgpu, let them
queue up.

GPU/Common/DrawEngineCommon.h

@@ -147,6 +147,8 @@ protected:
 
 	bool useHWTransform_ = false;
 	bool useHWTessellation_ = false;
+	// Used to prevent unnecessary flushing in softgpu.
+	bool flushOnParams_ = true;
 
 	// Vertex collector buffers
 	u8 *decoded = nullptr;