ALL: SDL1.2 to SDL2 migration

The main part is about the graphic stack and the remainder is about
events, the mousewheel and the textinput which is now distinct from
pressedkey.

A feature which would be nice to add is fullscreen from the desktop
resolution. SDL2 allows to do so with a single line.
GRAPHICS: Compute window maximum width and height
EVENTS: Fix enablement of repeated events
ALL: Fix the camera move

The camera should not move when the console is open. Since text input
and pressed key are now distinct events, we receive two events when a
key is actually pressed. So we have to test if text input is active.

One feature brings by SDL2 is the ability to recognize a key by is
position and not is name, that is to say we can ask the key next to
tab whatever the layout is (like qwerty, azerty or dvorak).
NWN: Avoid direct anti-aliasing change

In nwn, you can change the anti-aliasing "on the fly" with a slider.
As this change has to recreate the windows, we can miss the mouse up
event and the defocus of the slider. This commit apply the change only
if the ok button is pressed.
NWN: Properly initialize the fsaa value
GRAPHICS: Remember display when changing fsaa
NWN: Shut up a compiler warning by making _lastClickX/Y signed

Apparently, SDL_MouseButtonEvent.x/.y are signed in SDL2.
GRAPHICS: Shut up a compiler warning by commenting a variable

flags are unused in setFullScreen() and setScreenSize() at the moment.
DOCS: Note that we need SDL2
BUILD: Remove duplicate AC_SUBST()s for FreeType2
BUILD: Move SDL2 checking into ax_check_sdl2.m4
BUILD: Move libfaad checking into ax_check_faad.m4
BUILD: Move libmad checking into ax_check_mad.m4
BUILD: Move libogg/vorbisfile checking into ax_check_ogg.m4/vorbis.m4
BUILD: Move libxvidcore checking into ax_check_xvid.m4
BUILD: Move zlib checking into ax_check_zlib.m4
BUILD: Move iconv() into ax_check_iconv.m4
BUILD: Move OpenAL checking into ax_check_al.m4
BUILD: Move freetype2.m4 to ax_check_freetype2.m4