internal/shader: bug fix: need to covert constant type correctly for assignments

Closes #2934

internal/shader/stmt.go

@@ -582,6 +582,12 @@ func (cs *compileState) assign(block *block, fname string, pos token.Pos, lhs, r
 					cs.addError(pos, fmt.Sprintf("cannot use type %s as type %s in variable declaration", rts[i].String(), lts[i].String()))
 					return nil, false
 				}
+				switch lts[0].Main {
+				case shaderir.Int:
+					r[i].Const = gconstant.ToInt(r[i].Const)
+				case shaderir.Float:
+					r[i].Const = gconstant.ToFloat(r[i].Const)
+				}
 			}
 
 			if len(lhs) == 1 {

shader_test.go

@@ -2559,3 +2559,39 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2934
+func TestShaderAssignConst(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 {
+	a := 0.0
+	a = 1
+	b, c := 0.0, 0.0
+	b, c = 1, 1
+	d := 0.0
+	d += 1
+	return vec4(a, b, c, d)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w, h, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff}
+			if !sameColors(got, want, 2) {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}