egl: add EGL_MESA_device_software support

egl: add EGL_MESA_device_software support

Add a plain software device, which is always available.

We can safely assign it as the first/initial device in _eglGlobals,
although we ensure that's the case with a handful of _eglDeviceSupports
checks throughout the code.

v2:
 - s/_eglFindDevice/_eglAddDevice/ (Eric)
 - s/_eglLookupAllDevices/_eglRefreshDeviceList/ (Eric)
 - move ^^ helpers into a earlier patch (Eric, Mathias)
 - set the SW device on _eglGlobal init. (Eric)
 - add a number of _eglDeviceSupports checks (Mathias)
 - split Device/Display attach to a separate patch

v3:
 - flip inverted asserts (Mathias)
 - s/on-stack/static/ (Mathias)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>