ebiten: make sure panicking with a disposed shader

image.go

@@ -586,6 +586,10 @@ func (i *Image) DrawTrianglesShader(vertices []Vertex, indices []uint16, shader
 		return
 	}
 
+	if shader.isDisposed() {
+		panic("ebiten: the given shader to DrawTrianglesShader must not be disposed")
+	}
+
 	if len(vertices) > graphics.MaxVerticesCount {
 		// The last part cannot be specified by indices. Just omit them.
 		vertices = vertices[:graphics.MaxVerticesCount]
@@ -737,6 +741,10 @@ func (i *Image) DrawRectShader(width, height int, shader *Shader, options *DrawR
 		return
 	}
 
+	if shader.isDisposed() {
+		panic("ebiten: the given shader to DrawRectShader must not be disposed")
+	}
+
 	if options == nil {
 		options = &DrawRectShaderOptions{}
 	}

shader.go

@@ -57,6 +57,10 @@ func (s *Shader) Dispose() {
 	s.shader = nil
 }
 
+func (s *Shader) isDisposed() bool {
+	return s.shader == nil
+}
+
 // Deallocate deallocates the internal state of the shader.
 // Even after Deallocate is called, the shader is still available.
 // In this case, the shader's internal state is allocated again.

shader_test.go

@@ -2300,6 +2300,50 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 	}
 }
 
+func TestShaderDispose(t *testing.T) {
+	const w, h = 16, 16
+
+	dst := ebiten.NewImage(w, h)
+	s, err := ebiten.NewShader([]byte(`//kage:unit pixels
+
+package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	return vec4(1, 0, 0, 1)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w/2, h/2, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			var want color.RGBA
+			if i < w/2 && j < h/2 {
+				want = color.RGBA{R: 0xff, A: 0xff}
+			}
+			if got != want {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+
+	s.Dispose()
+
+	dst.Clear()
+
+	defer func() {
+		if e := recover(); e == nil {
+			panic("DrawRectShader with a disposed shader must panic but not")
+		}
+	}()
+
+	dst.DrawRectShader(w/2, h/2, s, nil)
+}
+
 func TestShaderDeallocate(t *testing.T) {
 	const w, h = 16, 16