Add screen magnifier

This adds a screen magnifier which can be controlled with the
`ZoomIn` / `ZoomOut` and `ToggleMagnify` actions.

It scales up part of the rendered framebuffer so the magnification
may end up looking blurry depending on the magnification scale.

PR #1774

src/meson.build

@@ -9,6 +9,7 @@ labwc_sources = files(
   'idle.c',
   'interactive.c',
   'layers.c',
+  'magnifier.c',
   'main.c',
   'node.c',
   'osd.c',