ebiten: Add more tests of unused variables in shaders

shader_test.go

@@ -526,6 +526,39 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	x := 0
 	return vec4(0)
 }
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := 0
+	x = 1
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := vec4(0)
+	x.x = 1
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+
+	if _, err := NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	x := 0
+	x++
+	return vec4(0)
+}
 `)); err == nil {
 		t.Errorf("error must be non-nil but was nil")
 	}