wacom: Fix potential warning on startup

When the window isn't setup yet, no need to try and invalidate
the whole drawing area.
wacom: Remove duplicate warning

gsd_wacom_osd_window_place_buttons() is the only caller, and
already warns about the inability to get the position for that button.
wacom: Make OSD work again

The recent lirsvg security changes made the include portion of the
SVG used for the OSD fail to parse.

Set the base URI for the layouts before loading the SVG data to
fix the new strict loading policy (#691708).

https://bugzilla.gnome.org/show_bug.cgi?id=709350