print UNDEFINED MODE over samples that are undefined in the GL spec

progs/demos/texenv.c

@@ -587,7 +587,18 @@ static void drawSample( int x, int y, int w, int h,
    glShadeModel( GL_FLAT );
    glDisable( GL_TEXTURE_2D );
 
-   if ( displayLevelInfo ) {
+   if ( envMode->mode == GL_DECAL &&
+        (format->baseFormat == GL_ALPHA ||
+         format->baseFormat == GL_LUMINANCE ||
+         format->baseFormat == GL_LUMINANCE_ALPHA ||
+         format->baseFormat == GL_INTENSITY)) {
+      /* undefined format/mode combination */
+      begin2D( w, h );
+      drawStringOutline( "UNDEFINED MODE", 15, h / 2,
+                         labelLevelColor0, labelLevelColor1 );
+      end2D();
+   }
+   else if ( displayLevelInfo ) {
       GLint width, height, border, components;
       GLint redSize, greenSize, blueSize, alphaSize;
       GLint luminanceSize, intensitySize;