ebiten: Error when the entrypoint is missing

Fixes #1323

shader.go

@@ -116,12 +116,22 @@ func NewShader(src []byte) (*Shader, error) {
 		return nil, err
 	}
 
-	// TODO: Create a pseudo vertex entrypoint to treat the attribute values correctly.
-	s, err := shader.Compile(fs, f, "__vertex", "Fragment", graphics.ShaderImageNum)
+	const (
+		vert = "__vertex"
+		frag = "Fragment"
+	)
+	s, err := shader.Compile(fs, f, vert, frag, graphics.ShaderImageNum)
 	if err != nil {
 		return nil, err
 	}
 
+	if s.VertexFunc.Block == nil {
+		return nil, fmt.Errorf("ebiten: vertex shader entry point '%s' is missing", vert)
+	}
+	if s.FragmentFunc.Block == nil {
+		return nil, fmt.Errorf("ebiten: fragment shader entry point '%s' is missing", frag)
+	}
+
 	return &Shader{
 		shader: mipmap.NewShader(s),
 	}, nil

shader_test.go

@@ -174,3 +174,10 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+func TestShaderNoMain(t *testing.T) {
+	if _, err := NewShader([]byte(`package main
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+}