Modified GIF background transparency

[heurist1]

Added a new parameter to use either background color or transparent

This is a solution for issue #73.
Sorry I can't see how to link this to the issue I raised, maybe I don't have the rights to do that.
I have attached the GIF that I wanted to use. I have trued it with both black and transparent background, but can see from the code that they will be treated the same.

For completenes the script I use is

kill_thread 1,0
reset
setup 1,512, 3
init
render
brightness 1,120
render

config_2D 1,32,16,2,32,8

thread_start 1
set_thread_exit_type 1,0
do
draw_image 1,/home/pi/snow03.gif,0,0,100,100,32,16,256,128,1,1,0
render 1
loop
thread_stop