compositor: Remove desktop zoom

Zoom is a neat trick, but in its current form it's very hard to test
and maintain.

It also causes output damage to scale outside of the output's boundaries,
which leads to an extra clipping step that's only necessary when zoom
is enabled.

Remove it to simplify desktop-shell and compositor.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -1068,7 +1068,7 @@ draw_paint_node(struct weston_paint_node *pnode,
 
 	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
 
-	if (pnode->view->transform.enabled || pnode->output->zoom.active ||
+	if (pnode->view->transform.enabled ||
 	    pnode->output->current_scale != pnode->surface->buffer_viewport.buffer.scale)
 		filter = GL_LINEAR;
 	else