Port sdl-display and sdl-draw example to SDL2

StefanBruens requested to merge StefanBruens/gegl:sdl2_port into master

The SDL2 Video API is split into several orthogonal parts, see https://wiki.libsdl.org/MigrationGuide#Video.

As SDL and SDL2 define the same symbols, only one of the two versions can be used. configure checks for SDL2 and SDL, preferring SDL2 if both are available.

The sdl-display op is duplicated as sdl2-display, applying the necessary changes.

The sdl-draw example is a straight port from SDL to SDL2.

Fixes #184

Edited by StefanBruens

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