Debug: Make it possible to save distance field cache on Windows

This makes compilation possible of the saveTexture() function which is
used to store the distance field glyph cache on disk when debugging.
It also changes the warnings output from here to have no prefix, since
this is anyway always available as the context in Qt 5.

Change-Id: If3b64f0921a59c485b5321a5465369708e764afc
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>