[GTK][Wayland] Implement support for running the layout tests under a (virtualized...

[GTK][Wayland] Implement support for running the layout tests under a (virtualized) Wayland environment.
https://bugs.webkit.org/show_bug.cgi?id=156716

Reviewed by Carlos Garcia Campos and Michael Catanzaro.

Tools:

* Scripts/webkitpy/layout_tests/run_webkit_tests.py: Allow to run the layout tests on Wayland by passing --wayland
(parse_args):
* Scripts/webkitpy/port/gtk.py:
(GtkPort.__init__):
(GtkPort._driver_class):
(GtkPort.setup_environ_for_server): Set the softgl renderer (now with EGL support) for the wayland tests.
(GtkPort._search_paths): When running inside the wayland environment, use ${port}-wayland as an additional
platform directory for storing the wayland specific layout test results and expectations for ${port}.
For example, the file LayoutTests/platform/gtk-wayland/TestExpectations will be used as the most specific
test expectations file on the GTK port when running the tests with the --wayland command line switch.
Baseline search path: gtk-wayland -> gtk -> wk2 -> generic.
* Scripts/webkitpy/port/westondriver.py: Run the weston display server inside Xvfb.
(WestonDriver.check_driver):
(WestonDriver.__init__):
(WestonDriver._start):
* Scripts/webkitpy/port/westondriver_unittest.py: Fix unittests for the new Weston inside Xvfb driver.
(WestonXvfbDriverDisplayTest):
(WestonXvfbDriverDisplayTest.__init__):
(WestonXvfbDriverDisplayTest._xvfb_run):
(WestonDriverTest.make_driver):
(WestonDriverTest.test_start):
* gtk/install-dependencies: Add the new dependencies that are required for either building weston,
or to satisfy the pkg-config check of the new added modules in JHBuild.
* gtk/jhbuild.modules: For building weston new enough on Debian Jessie we also need to build wayland
and libinput. Declare a pkg-config entry on all this modules to avoid building them if the ones
provided by the system are already new enough.
Switch the Mesa software rasterizer from a Xlib based GLX renderer to a DRI based EGL/GLX one.
It was the only way to get the software EGL rasterizer working without depending on user drivers.
Due to this the Mesa build now depends on a modern enough libdrm, so we also use JHBuild pkg-config
feature to avoid building libdrm when the one provided by the system is new enough.
* gtk/jhbuildrc:
* gtk/patches/xserver-search-for-DRI-drivers-at-LIBGL_DRIVERS_PATH-environ.patch: Added.
Xorg had no support for searching the DRI drivers on a custom path at runtime.
This patch implements support for that reusing the same variable that we use with Mesa to set the
custom path for the DRI drivers path.

LayoutTests:

* platform/gtk-wayland/TestExpectations: Added. Add initial Wayland specific test expectations.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@202619 268f45cc-cd09-0410-ab3c-d52691b4dbfc