Fixed seeking to an L frame from an RGB(A) GIF

[radarhere]

Resolves #6571

When pasting an L frame onto an RGB(A) GIF, convert the frame to RGB(A) first.

The test image was created using a modified Pillow, so that there is no global palette, and the image is comprised of a frame with a palette, and then two frames without. This means that the first frame is P mode, the second is L mode, combining to make RGB, and then there is an L mode frame pasted onto that RGB frame.