Add 'Add' methods

affine.go

@@ -35,6 +35,19 @@ func isIdentity(ebiten affine) bool {
 	return true
 }
 
+func add(lhs, rhs, result affine) {
+	dim := lhs.dim()
+	if dim != rhs.dim() {
+		panic("diffrent-sized matrices can't be multiplied")
+	}
+
+	for i := 0; i < dim-1; i++ {
+		for j := 0; j < dim; j++ {
+			result.setElement(i, j, lhs.Element(i, j)+rhs.Element(i, j))
+		}
+	}
+}
+
 func mul(lhs, rhs, result affine) {
 	dim := lhs.dim()
 	if dim != rhs.dim() {

colormatrix.go

@@ -60,6 +60,13 @@ func (c *ColorMatrix) Concat(other ColorMatrix) {
 	*c = result
 }
 
+// Add adds a color matrix with the other color matrix.
+func (c *ColorMatrix) Add(other ColorMatrix) {
+	result := ColorMatrix{}
+	add(&other, c, &result)
+	*c = result
+}
+
 func (c *ColorMatrix) setElement(i, j int, element float64) {
 	c.es[i][j] = element
 }

geometrymatrix.go

@@ -53,6 +53,13 @@ func (g *GeometryMatrix) Concat(other GeometryMatrix) {
 	*g = result
 }
 
+// Add adds a geometry matrix with the other geometry matrix.
+func (g *GeometryMatrix) Add(other GeometryMatrix) {
+	result := GeometryMatrix{}
+	add(&other, g, &result)
+	*g = result
+}
+
 func (g *GeometryMatrix) setElement(i, j int, element float64) {
 	g.es[i][j] = element
 }