[ImgBot] Optimize images

*Total -- 4.39kb -> 3.94kb (10.09%)

/textures/mapserver_poi_red.png -- 0.88kb -> 0.76kb (13.22%)
/textures/mapserver_poi_green.png -- 0.88kb -> 0.79kb (10.11%)
/textures/mapserver_poi_purple.png -- 0.87kb -> 0.79kb (9.43%)
/textures/mapserver_poi_orange.png -- 0.88kb -> 0.80kb (9%)
/textures/mapserver_poi_blue.png -- 0.88kb -> 0.80kb (8.67%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>