USAGE:
1. set background color
:call bgimg#set_bg(0xFFFFFF) " 0xRRGGBB
For Example:
:call bgimg#set_bg(str2nr(synIDattr(hlID('Normal'), 'bg#')[1:], 16))
2. set background image
:call bgimg#set_image("path/to/img.bmp")
Image will be drawn for background color you set.
Be aware of bitmap header version. LoadImage() doesn't support newer one.