[common] Screen size is auto-determined with SDL 2.0.3

Upstream changeset: https://hg.libsdl.org/SDL/rev/e33b5f7df761

The nemomobile package is SDL 2.0.3 from upstream plus that single
commit cherry-picked to allow proper operation also on Sailfish OS.
Merge pull request #1 from thp/auto-screen-size

[common] Screen size is auto-determined with SDL 2.0.3