sdl2: Update to version 2.0.20

sdl2: Update to version 2.0.20

- Update from 2.0.18 to 2.0.20
- Update of rootfile
- Changelog
   2.0.20:
    General:
     * SDL_RenderGeometryRaw() takes a pointer to SDL_Color, not int. You can cast color
       data in SDL_PIXELFORMAT_RGBA32 format (SDL_PIXELFORMAT_ABGR8888 on little endian
       systems) for this parameter.
     * Improved accuracy of horizontal and vertical line drawing when using OpenGL or
       OpenGLES
     * Added the hint SDL_HINT_RENDER_LINE_METHOD to control the method of line drawing
       used, to select speed, correctness, and compatibility.
    Windows:
     * Fixed size of custom cursors
    Linux:
     * Fixed hotplug controller detection, broken in 2.0.18

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>