overlay: allow to draw both/either of filled/outlined rectangle

Filled/outlined rectangles shown as snapping overlay are now enabled/disabled
independently with `snapping.overlay.[region|edge].bg.enabled` and
`snapping.overlay.[region|edge].border.enabled`.

To keep the default behavior, `*.bg.enabled` is yes and `*.border.enabled` is
no for hardware-based renderers, while `*.bg.enabled` is no and
`*.border.enabled` is yes for software-based (pixman) renderer.

Users can now use a filled rectangle as an overlay even with pixman renderer.
However, this may severely impact performance when used with translucent
`snapping.overlay.[region|edge].bg.color`.

This commit includes a refactor to use substruct `theme_snapping_overlay`
inside `theme` in order to pass it to `create_overlay_rect()` in a cleaner way.

Breaking changes is:
- `snapping.overlay.[region|edge].fill` is now removed.

src/main.c

@@ -171,7 +171,7 @@ main(int argc, char *argv[])
 	server_start(&server);
 
 	struct theme theme = { 0 };
-	theme_init(&theme, rc.theme_name);
+	theme_init(&theme, &server, rc.theme_name);
 	rc.theme = &theme;
 	server.theme = &theme;