[unity] Fixed SkeletonGraphic shader flickering on low alpha vertex colors in Linear color space. Closes #2128.

spine-unity/Assets/Spine/Runtime/spine-unity/Shaders/CGIncludes/Spine-Common.cginc

@@ -39,4 +39,18 @@ inline half4 PMAGammaToTargetSpace(half4 gammaPMAColor) {
 #endif
 }
 
+// Saturated version to prevent numerical issues that occur at CanvasRenderer
+// shader during linear-space PMA vertex color correction (countering automatic Unity conversion).
+// Note: Only use this method when the original color.rgb values lie within [0,1] range and
+// it's not an HDR color. This method is usually suitable for vertex color.
+inline half4 PMAGammaToTargetSpaceSaturated(half4 gammaPMAColor) {
+#if UNITY_COLORSPACE_GAMMA
+	return gammaPMAColor;
+#else
+	return gammaPMAColor.a == 0 ?
+		half4(GammaToLinearSpace(gammaPMAColor.rgb), gammaPMAColor.a) :
+		half4(saturate(GammaToLinearSpace(gammaPMAColor.rgb / gammaPMAColor.a)) * gammaPMAColor.a, gammaPMAColor.a);
+#endif
+}
+
 #endif

spine-unity/Assets/Spine/Runtime/spine-unity/Shaders/SkeletonGraphic/CGIncludes/Spine-SkeletonGraphic-NormalPass.cginc

@@ -49,7 +49,8 @@ VertexOutput vert (VertexInput IN) {
 #else
 	// Note: CanvasRenderer performs a GammaToTargetSpace conversion on vertex color already,
 	// however incorrectly assuming straight alpha color.
-	float4 vertexColor = PMAGammaToTargetSpace(half4(TargetToGammaSpace(IN.color.rgb), IN.color.a));
+	// Saturated version used to prevent numerical issues of certain low-alpha values.
+	float4 vertexColor = PMAGammaToTargetSpaceSaturated(half4(TargetToGammaSpace(IN.color.rgb), IN.color.a));
 #endif
 	OUT.color = vertexColor * float4(_Color.rgb * _Color.a, _Color.a); // Combine a PMA version of _Color with vertexColor.
 	return OUT;