[FIX] web_editor: get size of resizable elements on mouse down

Get the elements size also on mousedown. Because before that, the
size was only obtained on the start method when some elements did not
yet have its actual size. Example: elements in inactive slides in the
carousel snippet.

task-2162952

closes odoo/odoo#45217

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