stylecontext: Use right invalidation func

gtk_style_context_invalidate_internal() will respect only the current
saved state of the style context, which is wrong when updating the scale.
In that case, the whole style context needs updating.
stylecontext: Remove GTK_CSS_CHANGE_FORCE_INVALIDATE

This was introduced as a hackish way in 3.6 to make font updates
propagate properly. But since then, font handling has been changed and
this flag is no longer necessary.
widget: Make _gtk_set_simple_clip() take an optional content clip
label: Respect box-shadow when computing clip

Testclase included.

Also reorder headers to be alphabetic.
roundedbox: Only grow border-radius if there is one

This is relevant for shadow spread and goes in line with the web.

Testcase included.
css: Round shadow extents properly

Otherwise drawing will be clipped.

Testcase included
Use AC_USE_SYSTEM_EXTENSIONS to get _GNU_SOURCE, _XOPEN_SOURCE etc.

Similar to Bug #684123 in GLib.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen
Deprecate AM_PATH_GTK_3_0 in favour of PKG_CHECK_MODULES

That's what GNOME applications use.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen
Deprecate GTK_CHECK_BACKEND in favour of PKG_CHECK_MODULES

This is easy to do with pkg-config too.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen
Adwaita: inline toolbar buttons fixes

Properly styled single buttons in inline toolbars and fixed
a superflous border chiming in in backdrop.
Adwaita: some separators simplification

Accidentally fixing the backdrop separators in sidebars turning
black
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Adwaita: remove edge highlight form the assets

- using icon-shadow now, allows to treat special cases
  without introducing assets
Adwaita: no edge highlight for backdrop check/radios
Updated POTFILES.skip
Adwaita: try to be nicer on performances with entry shadows
GtkPopover: Fix rtl positioning

There was some confusion between unflipped and flipped positions.
Both final_position and current_position are meant to be unflipped,
and get_effective_position() needs to be applied to them to get
a flipped position. _gtk_window_set_popover_position() also expects
an unflipped position.

https://bugzilla.gnome.org/show_bug.cgi?id=735014
textbuffer: unit tests for the empty last line

For functions using _gtk_text_buffer_get_line_log_attrs():
- gtk_text_buffer_backspace()
- some gtk_text_iter functions (word/sentence/cursor boundaries)

As the FIXME comments show, there is a bug with
gtk_text_iter_is_cursor_position() for an empty last line.

https://bugzilla.gnome.org/show_bug.cgi?id=156164
Simplify _gtk_text_buffer_get_line_log_attrs()

NULL was returned in case of an empty last line. Every users needed to
special-case this. Now it will return the expected result: char_len of 0
with one PangoLogAttr.

In compute_log_attrs(), 'paragraph' will be the empty string "" with
'char_len' == 0.
pango_get_log_attrs() works fine with an empty string, it will return
one correct PangoLogAttr (because there is one text position for the
empty string).

It fixes the unit tests for gtk_text_iter_is_cursor_position().

https://bugzilla.gnome.org/show_bug.cgi?id=156164