gif2webp: Background color correction

For some GIF images, the first frame is missing the corresponding
graphic control extension. For such cases, we were never calling
GetBackgroundColor(), and default background color value (white) was being used
incorrectly.
So, we call GetBackgroundColor() when we encounter the first image
descriptor instead, to make sure that it is always called.

Change-Id: I00fc8e943d8a0c1578dcd718f3e74dec7de4ed61