[FIX] mass_mailing: ensure mails don't get distorted with DPI scaling

This ensure the layouts of emails don't get distorted in Outlook when
the user is using DPI scaling (typically, some elements get scaled up
while others don't).

X-original-commit: 2348efb6e22544d82ce53d542975f334d5553f62
Part-of: odoo/odoo#124465

addons/mass_mailing/data/mailing_data_templates.xml

@@ -15,6 +15,19 @@
                 <meta http-equiv="X-UA-Compatible" content="IE=9; IE=8; IE=7; IE=EDGE" />
 
                 <t t-call="mass_mailing.mass_mailing_mail_style"/>
+
+                <!--
+                    Prevent Outlook from distorting images with DPI scaling (see
+                    https://www.htmlemailcheck.com/knowledge-base/dpi-scaling-in-outlook-2007-2013/):
+                -->
+                <!--[if mso]>
+                    <xml>
+                        <o:OfficeDocumentSettings>
+                            <o:AllowPNG/>
+                            <o:PixelsPerInch>96</o:PixelsPerInch>
+                        </o:OfficeDocumentSettings>
+                    </xml>
+                <![endif]-->
             </head>
             <body>
                 <t t-out="body"/>