gloverlay: Fix upside down and miss-aligned JPEG

gloverlay: Fix upside down and miss-aligned JPEG

LibJPEG uses macroblock of 8x8 sample. In this element we use RGB and
Y444, two 24bit formats that are stored in 32bit pixels. This mean we
have 32x32 bytes macroblocks. For this reason, we need to allocate
our buffer slightly larger. We also need to pass the line pointer in
the right order, otherwise the image endup upside-down.

https://bugzilla.gnome.org/show_bug.cgi?id=745109