Avoid loading and keeping unused fallback font engines

Avoid loading and keeping unused fallback font engines

When we request glyphs from fallback fonts, we would potentially
load all fonts on the system into memory. This is especially true
for glyphs that are not supported by any font (or by the last in
the list) in any "Common" script (which e.g. includes CJK).

This would make any application which tried to display unsupported
glyphs use huge amounts of memory for keeping unused fonts cached,
only limited by the number of fonts on the system.

The patch contains two solutions: First, before loading the font,
the multi font engine will be asked whether it needs to be tried
for the given character. By default, this will always be true, so
all fonts will be tried, but with the new font config multi engine
in the platform plugin, it will ask FontConfig whether the font
contains a glyph for the character.

Should the font be loaded and still fail to resolve the character
(which could be the case for other platforms), we will simply
delete it again immediately instead keeping it cached.

Change-Id: I92dfb39289a359f49caa02c2caf8baf66098fb59
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>