Doesn't handle terminal content with background properly

[krzema12]

Consider e.g. the following content:https://tr1.cbsistatic.com/hub/i/2015/05/07/e4cf6574-f48a-11e4-940f-14feb5cc3d2a/whiptail_figure_1.png

Currently, all characters that have solid background are just omitted and not visible in the gravity-enabled mode.