Fixes firefox buggy rendering
Fixes firefox buggy rendering

In version 47 and 48, after switching tabs back and forth twice (or
maybe after a delay), and when dropping a piece on the board it would
not clear the areas around it from the hovering piece. This seems to be
solved in firefox 49 and 50.

luanlv/lila,luanlv/lila,clarkerubber/lila,arex1337/lila,arex1337/lila,arex1337/lila,luanlv/lila,luanlv/lila,arex1337/lila,luanlv/lila,arex1337/lila,clarkerubber/lila,clarkerubber/lila,clarkerubber/lila,luanlv/lila,clarkerubber/lila,clarkerubber/lila,clarkerubber/lila,luanlv/lila,arex1337/lila,arex1337/lila