Vsync support for DRM

[HidenoriMatsubayashi]

Add Vsync support for DRM backends.

Related issues

• Vsync support for Wayland #68
• Vsync support for X11 #137

[lhoward]

I'm looking into implementing this with drmWaitVBlank() – before I do it, is anyone able to give me an explanation of the advantages? I'm interested in anything that might help minimise CPU usage particularly with animations :)

cc @prspn @i2pi

Edit: after conferring with @i2pi it appears this is unlikely to improve performance, just potentially reduce tearing artefacts. See also here.