Adds a support for compiling flutter engine when
gdk does not have X11 backend. In such a configuration
the generated gdkconfig.h header file looks like the following:

 /* gdkconfig.h
  *
  * This is a generated file.  Please modify `configure.ac'
  */

 #ifndef __GDKCONFIG_H__
 #define __GDKCONFIG_H__

 #if !defined (__GDK_H_INSIDE__) && !defined (GDK_COMPILATION)
 #error "Only <gdk/gdk.h> can be included directly."
 #endif

 #include <glib.h>

 G_BEGIN_DECLS

 #define GDK_WINDOWING_WAYLAND

 G_END_DECLS

 #endif  /* __GDKCONFIG_H__ */

Additionally headers like <gdk/gdkx.h> are not available at all.

Above configuration can be found on the most of the embedded systems.

This patch enables compilation of X11 specific code only when gdk
defines GDK_WINDOWING_X11.
Signed-off-by: NDamian Wrobel <dwrobel@ertelnet.rybnik.pl>

* Revert "Fix documentation build for window changes. (#21780)"

This reverts commit 931a0468.

* Revert "Migration to PlatformDispatcher and multi-window (#20496)"

This reverts commit 85b0031f.

Extracts a TextRange class with a base and extent, and start(), end(),
collapsed(), and length() getters.

The possibility of reversed base and extent in selections and composing
ranges makes reasoning about them complex and increases the chances of
errors in the code. This change migrates most uses of base and extent in
the text model to start()/end() or position(). The position() method is
intended purely as an aid to readability to indicate that a collapsed
selection is expected at the call site; it also enforces a debug-time
assertion that this is the case.

The space in the argument name was causing this argument to be dropped and the sound and unsound dills to be identical.

* carrying code

* more changes for carrying the code

* rebase changes onto ios-screenshot tests

* adding screenshot capability to text_editing e2e test

* address some comments

* change enable flag for isUnitTestsScreenshotsAvailable

* addressing the reviewer comments

* change the dependency for path

* add to licencense file

* changing goldens commit no. the new commit has the screenshot goldens

* update readme file

* firefox tests needs LUCI changes

* change to release mode since screenshots were taken in release mode

* change window size

* some argument changes

* small comment change

* test the chrome linux tests again

* use roboto font instead of default font

* addressing reviewer comments

* change commit for goldens

* Fix documentation build for window changes.

* Add missing interfaces for web_ui

This reverts commit 5585ed99.

Additionally, the following _flutter.runInView deadlock is fixed.

Previously, a deadlock would occur when service protocol
_flutter.runInView is used to restart the engine wihtout tearing down
the shell: the shared mutex of the service protocol will be locked
during the restart as it's in the middle of handling a service protocol
message; if ServiceProtocol::AddHandler is also called during the
restart, the deadlock happens as AddHandler also requires such lock.

test/integration.shard/background_isolate_test.dart would fail
without this fix.

This is to unblock a Dart -> engine roll.

This is a PR for converting the dart:ui code in the engine to use a multi-window API. The goal here is to convert from the window singleton to an API that has the concept of multiple windows. Also, I'm matching up the new PlatformDispatcher class to talk directly to the PlatformConfiguration class in the engine. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows. The multi-window API in this PR currently only ever creates one window.

The design doc for this change is here.

The major changes in this PR:

Move the platfom-specific attributes out of Window, and into the new PlatformDispatcher class that holds all of the platform state, so that the platform code need only update the configuration on this class.
Create FlutterView, FlutterWindow, and SingletonFlutterWindow classes to separate out the concepts of a view (of which there may be multiple in a window), a window (of which there may be multiple on a screen, and they host views), and a window where there is only ever expected to be one (this hosts the entire API of the former Window class, and will eventually be the type of the window singleton).
Next step after this PR lands:

Remove the Window class entirely (it is replaced by SingletonFlutterWindow). Some minor changes in the Framework are needed to switch to using SingletonFlutterWindow directly first.

The Window class still exists in this PR, but will be removed as soon as the framework is converted to point to the SingletonFlutterWindow class instead. They share the same API, just have different names (Window is currently a subclass of SingletonFlutterWindow). The intention is that the Window name will be freed up to use as a widget class name in the framework for managing windows. The singleton called window will remain, and keep the same API it has now.

* Move shader.dart into shaders directory
* Add Shader builder basic structure and decls
* rewrite drawVertices with ShaderBuilder
* Fix in parameters in fragment shader to use varying for webgl1