Text shadows

[Kai Ma]

Hi emacs

Recently, I implemented the “text shadows” feature for Cairo-powered windowing 
systems, including X11 and Wayland.  See [1] if you wonder what that looks like.

This feature adds a new face attribute :shadow, and accepts values like:

- FLOAT                 how much blurring?
- (FLOAT . COLOR)       additionally, specifies a color for the shadow
- (FLOAT COLOR OFFSET)  additionally, specifies an offset (x . y)

This was first implemented for the NS port three years ago (also by me), and 
seems to have sparked some interest and was discussed on emacs-devel [2].  (I 
didn’t follow the discussion on emacs-devel back then.)

I’m writing to ask if the community is interested in accepting this feature 
into mainline Emacs?  If so, I will improve things and make a formal patch.

Also, I’d like some feedback on the design and the implementation [3].  The 
Gaussian blurring function used GCC vector extensions.  Is this allowed in the 
Emacs code?

There is also a known issue regarding text extents (?), and I’d like to know 
what is the correct path forward:

1. The shadows sometimes should be drawn outside the glyph extents, but they 
aren’t drawn.
2. Previously drawn shadows are not cleared, and they even accumulate.  This 
can be observed with blink-cursor-mode enabled.

Any input would be greatly appreciated!

[1] https://github.com/ksqsf/emacsmoe/pull/1#issuecomment-1465072298
[2] https://lists.gnu.org/archive/html/emacs-devel/2020-05/msg02921.html
[3] https://github.com/ksqsf/emacsmoe/pull/1/files 

— Kai