shared: Fix release/debugoptimized build warning with gcc

Fix ‘image’ clobbered by ‘longjmp’ warning using volatile.

jpeg_image_data members are now also declared volatile (like
png_image_data members) to explicitly request compilers to store them
on the stack (not in registers) to avoid warnings and bugs later.

Co-authored-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>