Make QtQuick2 compile on QPA

Make QtQuick2 compile on QPA

Moved the logic to set pixel size into the font engines to avoid
making the platform plugin interface too complex, and added a function
in QPA to make an isolated font engine based on font data. Currently
none of the QPA back-ends supports it, but it compiles and spits out
a warning if you try to create a QRawFont from data there. This isn't
used in QtQuick2 anyway.

Reviewed-by: Jiang Jiang