…other elements

Squashed commit of the following:

commit 3637dd92783bf0365953b4fd9c45dab932ec96f1
Author: William Light <git@wrl.lhiaudio.com>
Date:   Wed Oct 28 03:14:26 2020 +0100

    examples/test: overlay dragging demo

commit 5919892ac0dbed8a91d1be62afc87feb95c134c0
Author: William Light <git@wrl.lhiaudio.com>
Date:   Wed Oct 28 03:01:23 2020 +0100

    fix overlay blanking

commit f9b08be078ed2aa7f06eed409c5af3fc37174605
Author: William Light <git@wrl.lhiaudio.com>
Date:   Wed Oct 28 02:43:15 2020 +0100

    overlay mouse event support

commit 82ab14ad66c62745e968a190f030ed45131ae1f4
Author: William Light <git@wrl.lhiaudio.com>
Date:   Wed Oct 28 01:49:06 2020 +0100

    window: separate overlay surface

commit 8d6c8c8890dca286f17a2934daf762b32dfadfa4
Author: William Light <git@wrl.lhiaudio.com>
Date:   Wed Oct 28 00:58:05 2020 +0100

    overlays being drawn but no mouse interaction yet

commit e79c5d122f64eb0a0dd4062b2ea1c5dcdd626d96
Author: William Light <git@wrl.lhiaudio.com>
Date:   Tue Oct 27 23:50:54 2020 +0100

    surface: not sure why this was a separate definition

commit 89c37e550f9f35fe553ba98d000426b06053d334
Author: William Light <git@wrl.lhiaudio.com>
Date:   Tue Oct 27 23:48:13 2020 +0100

    fix implicit fall-through