MagickWand: Image layer merging doesn't work as expected with .psd, temporarily disabled for those files

This is due to a bug in ImageMagick, I believe. I haven't found a proper
way to check for .psd files except for comparing the file extension, so
I do that right now.
Pass error pointers to the buffered_file methods

This is required to correctly detect aborted I/O operations.
Image loader: Load new images before unloading old ones

This has some impact on file buffers: If the old images are unloaded
before the new image is loaded, a refcounter might decrease to zero and
therefore invalidate the buffer, initiating unnecessary I/O for
multi-page images.
Use GLib's slice allocator for file_t structures
Fix spectre function signature error introduced with commit 7356250
Merge branch 'master' into random_toggle_2

Conflicts:
	backends/spectre.c