[FIX] tools: fix image processing of palette PNG with alpha channel
[FIX] tools: fix image processing of palette PNG with alpha channel

Since b0ff033d695ed7da2ae5c6c08735ed09da70324e

To reproduce this issue: give to the processing method a PNG with transparency
and which is already encoded in palette mode.

In 12.0 such image could be `addons/payment/static/img/codensa_easy_credit.png`.

In earlier versions of Pillow, adding an alpha channel to such image would have
no effect, but since version 6.0.0 they raise an exception when that happens.

The PNG spec forbids the use of a full alpha channel with palette-based images
according to http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2

task-1998639

closes odoo/odoo#33628

Signed-off-by: Jérémy Kersten (jke) <56923a0c9ca793c07ff1df1013de5694062a27a6@openerp.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo