Add option to set window background color

This window background color is different than window.clear(color)
because this window color is the background color if the window itself,
which means if there is a delay between creating the window and the
first frame display then the window background color is black,
but if window color is set then that color will be displayed instead.
Normally you want to set the window background color to the same
color as the color you use in window.clear(color).

1 files changed, 0 insertions, 0 deletions

diff --git a/depends/mgl b/depends/mgl
-Subproject bb770991b2d688b247106e116c1ee222c6dbbe1
+Subproject c6d09be0a56c7b96be48f074901a53d7f76c25c