fixed r_lightmap affecting non-opaque surfaces

code/renderer/tr_shader.cpp

@@ -2128,10 +2128,20 @@ static shader_t* FinishShader()
 				stages[i].bundle.image[0] = tr.whiteImage;
 			}
 		}
-	} else if ( r_lightmap->integer ) {
+	} else if (
+		r_lightmap->integer &&
+		( shader.contentFlags & CONTENTS_TRANSLUCENT ) == 0 &&
+		shader.sort <= SS_OPAQUE ) {
 		// we reduce it down to a single lightmap stage with the same state bits as
 		// the current first stage (if the shader uses a lightmap at all)
 
+		// @NOTE: we ignore all surfaces that aren't fully opaque because:
+		// a) it's hard to know what level of uniform opacity would emulate the original look best
+		// b) alpha-tested surfaces that have been turned into alpha-blended ones can't just use any given sort key:
+		//    you can always find or design a case that won't work correctly
+		// for decals entirely pressed against opaque surfaces, we could use a keyword ("polygonoffset" or something new)
+		// to know that we should not draw them at all, but we just shouldn't trust level designers
+
 		// look for "real" lightmaps first (straight from the .bsp file itself)
 		int stageIndex = -1;
 		for ( int i = 0; i < shader.numStages; ++i ) {