Fix tint op by ensuring LAB and allowing negative values #1235

Add test cases for more tint colours and input interpretations

src/pipeline.h

@@ -157,8 +157,8 @@ struct PipelineBaton {
     cropOffsetLeft(0),
     cropOffsetTop(0),
     premultiplied(false),
-    tintA(0.0),
-    tintB(0.0),
+    tintA(128.0),
+    tintB(128.0),
     flatten(false),
     negate(false),
     blurSigma(0.0),