Default to only use ARGB visual for ARGB client windows

[yshui]

Don't use the same visual for all windows.

And add a --full-argb option to force ARGB visual for all windows, which is the current default behavior.

Fixes #2408

[yshui]

Maybe it's better to instead keep the current default, and enable the new behavior with an option, like --smart-argb or something :thinking:

[Elv13]

Sorry for the delay, I don't have a lot of time for Awesome right now. Won't think break people who add transparent titlebars or use anti-aliased round corners?

[yshui]

@Elv13 yes, this will. alternatively we can also keep the default and add an option to enable this behavior, maybe --argb-if-needed.

[actionless]

yup, i also think it's better to preserve the current behavior (forced argb) as a default, at least til next major release

[Elv13]

@yshui Maybe deprecate --no-argb and add a --argb-mode=<on|off|ondemand> where on remains the default?

* ondemand and not auto because auto would imply it detects when it really needs it, which we cannot really do sanely because the users are free to render whatever they want in the titlebars.