bug#22549: 25.0.50; Tooltips placed outside of screen limits

[Óscar Fuentes]

emacs -Q
M-x toggle-frame-fullscreen

Place the mouse pointer on some element of the mode line and wait for
the tooltip. The tooltip frame is partially invisible because its top
left coordinates are determined from the mouse pointer position and, as
it is positioned near the bottom of the screen, there is no room left
for the tooltip.

A tooltip may also be partially hidden because it has no enough
horizontal space when the mouse pointer is near the right edge of the
screen.

The MS Windows port places the tooltips taking into account the screen
edges.

In GNU Emacs 25.0.50.29 (x86_64-unknown-linux-gnu, X toolkit)
 of 2016-01-21 built on qcore
Repository revision: 5293d1bdb3e665a565af032a163004c976328cc2
Windowing system distributor 'The X.Org Foundation', version 11.0.11702000
System Description:     Ubuntu 15.10

Configured using:
 'configure --without-toolkit-scroll-bars --with-x-toolkit=lucid'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG SOUND GSETTINGS NOTIFY GNUTLS LIBXML2
FREETYPE XFT ZLIB LUCID X11

Important settings:
  value of $LANG: C
  locale-coding-system: nil