st/texture-cache: Add fallback icon for property binds

X11 windows of window backed applications that do not have an icon set
by the time the app tracker is first asked to create an icon texture for
them, will never have a (visible) icon texture. This is due to the
surface behind the MetaWindow::icon property not being an image surface,
leading st_texture_cache_reset_texture to create the StImageContent with
a preferred size of 0, which means that all following updates to the
icon will done with this invisible StImageContent.

Also there was no fallback icon set for the cases that never receive an
icon in the window backed X11 case.

This fixes both cases by creating an image surface using a specified
fallback icon when no valid surface can be found.
11 jobs for !1711 with fix-missing-fallback-icon-x11 in 4 minutes and 21 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Pre Review
passed #1174065
check_commit_log

00:01:10

passed #1174066
check-merge-request

00:00:27

 
  Prep
passed #1174067
build-fedora-container

00:00:47

 
  Review
passed #1174069
eslint

00:01:02

passed #1174070
eslint_mr

00:00:45

passed #1174068
js_check

00:00:46

passed #1174072
no_template_check

00:00:22

passed #1174071
potfile_check

00:00:46

 
  Build
passed #1174073
build

00:02:15

passed #1174074
flatpak
flatpak

00:02:58

 
  Test
passed #1174075
test

00:00:54