use glScissor() instead of glViewport() when drawing widgets

src/Widget.cc

@@ -11,7 +11,7 @@ static VALUE Widget_render_begin(VALUE self)
     int width = FIX2INT(rb_iv_get(self, "@width"));
     int height = FIX2INT(rb_iv_get(self, "@height"));
 
-    glViewport(x, y, width, height);
+    glScissor(x, y, width, height);
 
     return Qnil;
 }

src/Window.cc

@@ -85,6 +85,8 @@ static void Init_OpenGL()
         rb_raise(rb_eRuntimeError, "OpenGL 3.0 is not supported");
     }
 
+    glEnable(GL_SCISSOR_TEST);
+
     initialized = true;
 }