xcf: Properly read image resolution

Summary:
QDataStream reads 64 bits when reading into a float unless you tell it to use SinglePrecision,
since floats in xcf are 32 bit, do that

Reviewers: cfeck, apol, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24113