entry: Restrict magnifier popover within the entry size

The magnifier shown with touch input should not go beyond the widget
limits.
7 jobs for wip/sadiq/fix-entry-magnifier in 22 minutes and 36 seconds
Status Job ID Name Coverage
  Build
passed #807495
debian-meson

00:04:58

passed #807492
fedora-autotools

00:10:26

manual #807493
allowed to fail manual
fedora-distcheck
passed #807494
fedora-meson

00:09:46

manual #807497
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #807496
win32-ps
msys2-mingw64-meson

00:08:53

 
  Docs
passed #807498
reference

00:12:10