ebiten: Bug fix: Colorm.Concat crashed when the argument is an initial value

Closes #1765

colorm.go

@@ -64,7 +64,11 @@ func (c *ColorM) Apply(clr color.Color) color.Color {
 // Concat multiplies a color matrix with the other color matrix.
 // This is same as muptiplying the matrix other and the matrix c in this order.
 func (c *ColorM) Concat(other ColorM) {
-	c.impl = c.affineColorM().Concat(other.impl)
+	o := other.impl
+	if o == nil {
+		o = affine.ColorMIdentity{}
+	}
+	c.impl = c.affineColorM().Concat(o)
 }
 
 // Scale scales the matrix by (r, g, b, a).

image_test.go

@@ -2426,3 +2426,13 @@ func BenchmarkColorMScale(b *testing.B) {
 		dst.DrawImage(src, op)
 	}
 }
+
+// #1765
+func TestColorMConcat(t *testing.T) {
+	var a, b ColorM
+	a.SetElement(1, 2, -1)
+	a.Concat(b)
+	if got, want := a.Element(1, 2), -1.0; got != want {
+		t.Errorf("got: %f, want: %f", got, want)
+	}
+}