sdl2: Update to version 2.0.18

sdl2: Update to version 2.0.18

- Update from 2.0.16 to 2.0.18
- Update of rootfile
- Changelog
   2.0.18
    General:
     * The SDL wiki documentation and development headers are automatically kept in sync
     * Each function has information about in which version of SDL it was introduced
     * Added the hint SDL_HINT_APP_NAME to let SDL know the name of your application for
       various places it might show up in system information
     * Added SDL_RenderGeometry() and SDL_RenderGeometryRaw() to allow rendering of
       arbitrary shapes using the SDL 2D render API
     * Added SDL_SetTextureUserData() and SDL_GetTextureUserData() to associate
       application data with an SDL texture
     * Added SDL_RenderWindowToLogical() and SDL_RenderLogicalToWindow() to convert
       between window coordinates and logical render coordinates
     * Added SDL_RenderSetVSync() to change whether a renderer present is synchronized
       with vblank at runtime
     * Added SDL_PremultiplyAlpha() to premultiply alpha on a block of
       SDL_PIXELFORMAT_ARGB8888 pixels
     * Added a window event SDL_WINDOWEVENT_DISPLAY_CHANGED which is sent when a window
       changes what display it's centered on
     * Added SDL_GetWindowICCProfile() to query a window's ICC profile, and a window
       event SDL_WINDOWEVENT_ICCPROF_CHANGED that is sent when it changes
     * Added the hint SDL_HINT_VIDEO_EGL_ALLOW_TRANSPARENCY to allow EGL windows to be
       transparent instead of opaque
     * SDL_WaitEvent() has been redesigned to use less CPU in most cases
     * Added SDL_SetWindowMouseRect() and SDL_GetWindowMouseRect() to confine the mouse
       cursor to an area of a window
     * You can now read precise mouse wheel motion using 'preciseX' and 'preciseY' event
       fields
     * Added SDL_GameControllerHasRumble() and SDL_GameControllerHasRumbleTriggers() to
       query whether a game controller supports rumble
     * Added SDL_JoystickHasRumble() and SDL_JoystickHasRumbleTriggers() to query whether
       a joystick supports rumble
     * SDL's hidapi implementation is now available as a public API in SDL_hidapi.h
    Windows:
     * Improved relative mouse motion over Windows Remote Desktop
     * Added the hint SDL_HINT_IME_SHOW_UI to show native UI components instead of hiding
       them (defaults off)
    Windows/UWP:
     * WGI is used instead of XInput for better controller support in UWP apps
    Linux:
     * Added the hint SDL_HINT_SCREENSAVER_INHIBIT_ACTIVITY_NAME to set the activity
       that's displayed by the system when the screensaver is disabled
     * Added the hint SDL_HINT_LINUX_JOYSTICK_CLASSIC to control whether /dev/input/js*
       or /dev/input/event* are used as joystick devices
     * Added the hint SDL_HINT_JOYSTICK_DEVICE to allow the user to specify devices that
       will be opened in addition to the normal joystick detection
     * Added SDL_LinuxSetThreadPriorityAndPolicy() for more control over a thread
       priority on Linux
    Android:
     * Added support for audio output and capture using AAudio on Android 8.1 and newer
     * Steam Controller support is disabled by default, and can be enabled by setting the
       hint SDL_HINT_JOYSTICK_HIDAPI_STEAM to "1" before calling SDL_Init()
    Apple Arcade:
     * Added SDL_GameControllerGetAppleSFSymbolsNameForButton() and
       SDL_GameControllerGetAppleSFSymbolsNameForAxis() to support Apple Arcade titles
    iOS:
     * Added documentation that the UIApplicationSupportsIndirectInputEvents key must be
       set to true in your application's Info.plist in order to get real Bluetooth mouse
       events.
     * Steam Controller support is disabled by default, and can be enabled by setting the
       hint SDL_HINT_JOYSTICK_HIDAPI_STEAM to "1" before calling SDL_Init()

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>