[IMP] test_themes, theme_*: adapt themes for new parallax structure

This commit adapts the themes to the new Parallax effect structure.
We now wrap the parallax element in another element with
"overflow: hidden" to avoid setting "overflow: hidden" on sections.
That was causing elements like the dropdown of the search bar to be cut
off and not fully visible.

task-4926420

closes odoo/design-themes#1114

Related: odoo/odoo#218296
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

theme_bistro/views/snippets/s_image_title.xml

@@ -9,7 +9,9 @@
         <attribute name="data-scroll-background-ratio">-3</attribute>
     </xpath>
     <xpath expr="//div[hasclass('o_we_bg_filter')]" position="before">
-        <span class="s_parallax_bg oe_img_bg o_bg_img_center" style="background-image: url('/web/image/website.s_image_title_default_image') !important;"/>
+        <span class="s_parallax_bg_wrap">
+            <span class="s_parallax_bg oe_img_bg o_bg_img_center" style="background-image: url('/web/image/website.s_image_title_default_image') !important;"/>
+        </span>
     </xpath>
 
     <!-- Texts -->

theme_bistro/views/snippets/s_product_catalog.xml

@@ -9,7 +9,9 @@
         <attribute name="data-scroll-background-ratio">1</attribute>
     </xpath>
     <xpath expr="//div[hasclass('o_we_bg_filter')]" position="replace">
-        <span class="s_parallax_bg oe_img_bg" style="background-image: url('/web/image/website.s_product_catalog_default_image'); background-position: 50% 0%;"/>
+        <span class="s_parallax_bg_wrap">
+            <span class="s_parallax_bg oe_img_bg" style="background-image: url('/web/image/website.s_product_catalog_default_image'); background-position: 50% 0%;"/>
+        </span>
         <div class="o_we_bg_filter bg-black-50"/>
     </xpath>
     <!-- Remove Title & lead -->