Fix: virtual call from constructor

That fills an instance variable that is only read from the Game Options window
and that is overwritten when the video driver is started. Since you cannot get
into the Game Options window without starting the video driver, it is just
pointless and wrong code that would never be noticed by the end user.

Codechange: use _cur_palette the same in all the drivers

It was a bit of a mixed bag. With this change, gfx.cpp is in
control who accesses _cur_palette from the video-drivers.

Remove: [Video] no longer draw in a thread

Drawing in a thread is a bit odd, and often leads to surprising
issues. For example, OpenGL would only allow it if you move the
full context to the thread. Which is not always easily done on
all OSes.
In general, the advise is to handle system events and drawing
from the main thread, and do everything else in other threads.
So, let's be more like other games.

Additionally, putting the drawing routine in a thread was only
done for a few targets.

Upcoming commit will move the GameLoop in a thread, which will
work for all targets.

Codechange: [Video] make the prototype of PollEvent() the same for all drivers

Additionally, call it from the draw-tick.

Codechange: [SDL2] Allow several places to hook into the SDL driver

This allows future subdrivers to use these to manage their
own flow.

Codechange: [SDL2] Move functions and variables to class-scope

This allows future subdrivers to override them.

Codechange: [SDL2] Move SDLSurface code to its own function

This increases readability, and allow future subdrivers to not
use SDLSurface to draw.

Codechange: [SDL2] Split Start() in a few more functions

This makes it a bit easier to follow what is going on, and
allow future subdrivers to hook into a few of these functions.

Reworked the code slighly while at it, to return early where
possible.

Codechange: deduplicate tick-handlers of all video drivers

They were all identical, so better put this in a single place
hoping it is less likely to break.

Codechange: be consistent in what CheckPaletteAnim() does and when it is called

Additionally, make sure this is a class method. Later commits
will make use of this.

Codechange: be consistent in naming the paint function Paint()

Also move this function to be a class member. This to allow
further deduplicating of code in a later commit.

Add: draw the screen at a steady pace, also during fast-forward

During fast-forward, the game was drawing as fast as it could. This
means that the fast-forward was limited also by how fast we could
draw, something that people in general don't expect.

To give an extreme case, if you are fully zoomed out on a busy
map, fast-forward would be mostly limited because of the time it
takes to draw the screen.

By decoupling the draw-tick and game-tick, we can keep the pace
of the draw-tick the same while speeding up the game-tick. To use
the extreme case as example again, if you are fully zoomed out
now, the screen only redraws 33.33 times per second, fast-forwarding
or not. This means fast-forward is much more likely to go at the
same speed, no matter what you are looking at.

Codechange: track _realtime_tick more accurate

_realtime_tick was reset every time the diff was calculated. This
means if it would trigger, say, every N.9 milliseconds, it would
after two iterations already drift a millisecond. This adds up
pretty quick.

Codechange: switch all video drivers to std::chrono for keeping time

On all OSes we tested the std::chrono::steady_clock is of a high
enough resolution to do millisecond measurements, which is all we
need.

By accident, this fixes a Win32 driver bug, where we would never
hit our targets, as the resolution of the clock was too low to
do accurate millisecond measurements with (it was ~16ms resolution
instead).

Codechange: [SDL2] Rework how palette is updated

It now follows more what the Win32 driver does, and has far less
exceptions and special casing.

MakePalette creates the Palette and prepares surface.
UpdatePalette updates the Palette.
CheckPaletteAnim checks if UpdatePalette needs to be called and
marks the whole screen dirty so DrawSurfaceToScreen will do a
full redraw.

Codechange: [SDL2] Split away CreateMainWindow from CreateMainSurface

This makes the code a bit more readable, as both intentions are
more clear, and there is less nesting in the main function.

Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

Codechange: unroll the SDL2 main loop

This commit prepares for the next commit, as Emscripten needs to
have a way to trigger a single iteration of the main loop. To
keep the real changes more clear, this commit only unrolls the
loop, and makes no changes to the logic itself.

Fix #7783, Fix #7816: [SDL2] Fix input handling in edit context

In particular this fixes handling of the shift key