shape: Add a "solid shadow" shape.

[Elv13]

Credit to @actionless for originally using it in some of his themes.

In my case, I originally wrote it for this MS-DOS homage demo https://imgur.com/a/8ikCPl0

I upstream it now because it got some little interest in the chat.

[codecov[bot]]

Codecov Report

Merging #3489 (9797e47) into master (1e0d1e7) will decrease coverage by 0.01%. The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master    #3489      +/-   ##
==========================================
- Coverage   90.25%   90.24%   -0.02%     
==========================================
  Files         822      823       +1     
  Lines       53561    53600      +39     
==========================================
+ Hits        48341    48370      +29     
- Misses       5220     5230      +10     

Flag	Coverage Δ
gcov	90.24% <95.00%> (-0.02%)	:arrow_down:
luacov	93.30% <95.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/gears/shape.lua	97.87% <93.93%> (-0.38%)	:arrow_down:
...ts/examples/gears/shape/solid_rectangle_shadow.lua	100.00% <100.00%> (ø)
event.c	71.94% <0.00%> (-1.21%)	:arrow_down:
lib/wibox/widget/imagebox.lua	94.64% <0.00%> (-0.90%)	:arrow_down:
spawn.c	86.09% <0.00%> (-0.07%)	:arrow_down:

[actionless]

i wonder how it could be used for that practically?

in my case the client just had 4 titlebars and each of them was handling drawing 1/4 of the shape on its own

[actionless]

i meant, from the point of code, how to set this shape to be a different-color shadow of the titlebar

[Elv13]

@actionless The wibox.container.borders module in the PR above makes this more usable. While it's not merged yet[1], is that example use case enough to merge this now? It isn't a super huge change.

[1] (3rd item in my slowly progressing TODO list after fixing the CI and getting the doc reactor branch ready),