layout: Try to allocate before getting size of tracked actors

Because we're about to `get_transformed_{position,size}` of each,
which will return NaNs if not yet allocated. Those NaNs were finding
their way into the workspace strut definitions on startup and not
getting corrected until after the startup animation completed. This
meant any extensions depending on the `workareas-changed` signal were
getting an incorrect workarea (the whole workspace) and so were
rendered out of place during the login animation. Now they're not.

Fixes: GNOME/mutter#1627 and
       https://launchpad.net/bugs/1919979
11 jobs for !1785 with fix-lp1919979 in 5 minutes and 6 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Pre Review
passed check_commit_log #1242387

00:00:34

passed check-merge-request #1242388

00:00:21

 
  Prep
passed build-fedora-container #1242389

00:00:28

 
  Review
passed eslint #1242391

00:01:20

passed eslint_mr #1242392

00:00:48

passed js_check #1242390

00:00:23

passed no_template_check #1242394

00:00:22

passed potfile_check #1242393

00:00:27

 
  Build
passed build #1242395

00:03:18

passed flatpak #1242396
flatpak

00:01:30

 
  Test
passed test #1242397

00:01:12