PopplerRectangle alloc/free mismatch

When e3fed321 internally extended the PopplerRectangle type into PopplerRectangleExtended, the allocation was changed to g_slice_new0(PopplerRectangleExtended) but the free was left at g_slice_free(PopplerRectangle, rectangle);.

This confuses the slice allocator. It crashes loudly with G_SLICE=debug-blocks, while G_SLICE=always-malloc works around the bug as free does not care about allocation sizes.

This seems to be the underlying cause of https://gitlab.gnome.org/GNOME/evince/-/issues/1608.

/cc @nbenitez

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information