Support for .ico image format

[Orgoth]

I actually only have one question.
What exactly is the reason that GD-Lib does not support the .ico format. "https://de.wikipedia.org/wiki/ICO_(Dateiformat)"
The format is supported by all current browsers and also ImageMagick, only GD seems to be missing it.

The background is as follows.
I had a discussion with another developer who insisted that the format is no longer supported at all "HTML Spec, all Browsers, ImageMagick and GD-Lib" and a review of the whole thing showed that except for his statement about GD, all the others were wrong.

[vapier]

considering browsers allow favico.ico to be just about any format, and html has <link rel="icon" ...> tags for just about any format, the actual ico format isn't really that important.

that said, i'm not against adding support for the format since it isn't terribly complicated. patches w/unittest coverage welcome.