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.
...and unify region overlay and snap-to-edge overlay into overlay.c.
Snap-to-edge overlay is delayed for 500ms to prevent flickering when
the view is dragged from an output to another (demo in discussion labwc#1613).
This also fixes a bug that region overlay is not shown when a modifier
key is re-pressed.
