Added error message when the XShape extension isn't supported.

src/main.cpp

     if ( err != EXIT_SUCCESS ) {
         return EXIT_FAILURE;
     }
+    if ( !slop::isRectangleSupported() ) {
+        fprintf( stderr, "Error: Your X server doesn't support the XShape extension. There's nothing slop can do about this!\n" );
+        fprintf( stderr, "  Try updating X and making sure you have XExtensions installed. (/usr/lib/libXext.so, /usr/include/X11/extensions/shape.h)\n" );
+        return EXIT_FAILURE;
+    }
     int state = 0;
     bool running = true;
     slop::Rectangle* selection = NULL;

src/rectangle.cpp

     }
     return color;
 }
+
+bool slop::isRectangleSupported() {
+    int event_base;
+    int error_base;
+    return XShapeQueryExtension( xengine->m_display, &event_base, &error_base );
+}

src/rectangle.hpp

     void    constrain( int w, int h );
 };
 
+bool isRectangleSupported();
+
 }
 
 #endif // IS_RECTANGLE_H_