Enhancement: support adding (first) frame with transparency about cgif

[dloebl]

I had another look at this issue / your PR #5:
As far as I understand, transparency (that is visible to the user) is nothing the original GIF spec defines.
https://www.w3.org/Graphics/GIF/spec-gif89a.txt

However, it became the de-facto standard that decoders/image viewers treat the background as transparent. Areas of the first frame that are not defined by the image data are transparent. Disposal method 2 (restore to background) makes it possible to show visible transparency on the following frame(s) as well.

It seems that implementing transparency like this became the new standard. So we will implement it as you suggested 😄

[dloebl]

Support for user-defined transparency has been added (your PR #5) and can be set with the global / GIF level attribute GIF_ATTR_HAS_TRANSPARENCY.

[lovell]

Thank you / danke!