[FIX] tools.image: Allow conversion of P to JPEG

While upgrade of a customer database, A P-type image
was being converted to JPEG image. It didn't allow to save
as direct conversion from P-type(palette) to JPEG. So, fix
covers conversion of such corner cases to first convert it
to RGB and then RGB can convert it to desired output format
opw-3043418

closes odoo/odoo#105549

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>