This PR adds support for the _blink_ graphic rendition attribute. When a
character is output with this attribute set, it "blinks" at a regular
interval, by cycling its color between the normal rendition and a dimmer
shade of that color.
The majority of the blinking mechanism is encapsulated in a new
`BlinkingState` class, which is shared between the Terminal and Conhost
implementations. This class keeps track of the position in the blinking
cycle, which determines whether characters are rendered as normal or
faint.
In Windows Terminal, the state is stored in the `Terminal` class, and in
Conhost it's stored in the `CONSOLE_INFORMATION` class. In both cases,
the `IsBlinkingFaint` method is used to determine the current blinking
rendition, and that is passed on as a parameter to the
`TextAttribute::CalculateRgbColors` method when these classes are
looking up attribute colors.
Prior to calculating the colors, the current attribute is also passed to
the `RecordBlinkingUsage` method, which keeps track of whether there are
actually any blink attributes in use. This is used to determine whether
the screen needs to be refreshed when the blinking cycle toggles between
the normal and faint renditions.
The refresh itself is handled by the `ToggleBlinkingRendition` method,
which is triggered by a timer. In Conhost this is just piggybacking on
the existing cursor blink timer, but in Windows Terminal it needs to
have its own separate timer, since the cursor timer is reset whenever a
key is pressed, which is not something we want for attribute blinking.
Although the `ToggleBlinkingRendition` is called at the same rate as the
cursor blinking, we actually only want the cells to blink at half that
frequency. We thus have a counter that cycles through four phases, and
blinking is rendered as faint for two of those four. Then every two
cycles - when the state changes - a redraw is triggered, but only if
there are actually blinking attributes in use (as previously recorded).
As mentioned earlier, the blinking frequency is based on the cursor
blink rate, so that means it'll automatically be disabled if a user has
set their cursor blink rate to none. It can also be disabled by turning
off the _Show animations in Windows_ option. In Conhost these settings
take effect immediately, but in Windows Terminal they only apply when a
new tab is opened.
This PR also adds partial support for the `SGR 6` _rapid blink_
attribute. This is not used by DEC terminals, but was defined in the
ECMA/ANSI standards. It's not widely supported, but many terminals just
it implement it as an alias for the regular `SGR 5` blink attribute, so
that's what I've done here too.
## Validation Steps Performed
I've checked the _Graphic rendition test pattern_ in Vttest, and
compared our representation of the blink attribute to that of an actual
DEC VT220 terminal as seen on [YouTube]. With the right color scheme
it's a reasonably close match.
[YouTube]: https://www.youtube.com/watch?v=03Pz5AmxbE4&t=1m55sCloses#7388
d1671a0acd