glamor: make sure the correct FBO is cleared

This also removes an unnecesary call to glDrawBuffer.
Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Fixes: 0e9a0c20 - "xwayland: clear pixmaps after creation in rootless
                   mode"
Closes: xorg/xserver#933

glamor/glamor_fbo.c

@@ -253,7 +253,8 @@ glamor_pixmap_clear_fbo(glamor_screen_private *glamor_priv, glamor_pixmap_fbo *f
         glClearTexImage(fbo->tex, 0, pixmap_format->format, pixmap_format->type, NULL);
     }
     else {
-        glDrawBuffer(GL_COLOR_ATTACHMENT0); /* assumes fbo->fb was attached as GL_COLOR_ATTACHMENT0 */
+        glamor_set_destination_pixmap_fbo(glamor_priv, fbo, 0, 0, fbo->width, fbo->height);
+
         glClearColor(0.0, 0.0, 0.0, 0.0);
         glClear(GL_COLOR_BUFFER_BIT);
     }