message: More conveniently switch between old and new OpenGL
diff: #include <visionaray/gl/util.h>
  #include <visionaray/pixel_format.h>
  +#define VSNRAY_OPENGL_LEGACY 0
  +
  namespace visionaray
  {
  namespace gl
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  //-------------------------------------------------------------------------------------------------
  // Shader program to display color texture w/o depth compositing
  @@ -262,7 +264,7 @@ void depth_program::disable() const
  struct depth_compositor::impl
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  impl()
  : vertex_buffer(create_buffer())
  , tex_coord_buffer(create_buffer())
  @@ -320,7 +322,7 @@ struct depth_compositor::impl
  };
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  void depth_compositor::impl::set_texture_params() const
  {
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  @@ -346,7 +348,7 @@ depth_compositor::~depth_compositor()
  void depth_compositor::composite_textures() const
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLint active_texture = GL_TEXTURE0;
  GLuint bound_texture = 0;
  @@ -422,7 +424,7 @@ void depth_compositor::composite_textures() const
  void depth_compositor::display_color_texture() const
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLint active_texture = GL_TEXTURE0;
  GLuint bound_texture = 0;
  @@ -464,7 +466,7 @@ void depth_compositor::display_color_texture() const
  void depth_compositor::setup_color_texture(pixel_format_info info, GLsizei w, GLsizei h)
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLuint bound_texture = 0;
  glGetIntegerv(GL_TEXTURE_BINDING_2D, reinterpret_cast<GLint*>(&bound_texture));
  @@ -489,7 +491,7 @@ void depth_compositor::setup_color_texture(pixel_format_info info, GLsizei w, GL
  void depth_compositor::setup_depth_texture(pixel_format_info info, GLsizei w, GLsizei h)
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLuint bound_texture = 0;
  glGetIntegerv(GL_TEXTURE_BINDING_2D, reinterpret_cast<GLint*>(&bound_texture));
  @@ -519,7 +521,7 @@ void depth_compositor::update_color_texture(
  GLvoid const* data
  ) const
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLuint bound_texture = 0;
  glGetIntegerv(GL_TEXTURE_BINDING_2D, reinterpret_cast<GLint*>(&bound_texture));
  @@ -547,7 +549,7 @@ void depth_compositor::update_depth_texture(
  GLvoid const* data
  ) const
  {
  -#if !defined(VSNRAY_OPENGL_LEGACY)
  +#if !VSNRAY_OPENGL_LEGACY
  // Store OpenGL state
  GLuint bound_texture = 0;
  glGetIntegerv(GL_TEXTURE_BINDING_2D, reinterpret_cast<GLint*>(&bound_texture));
  