libsdl2: upgrade 2.0.18 -> 2.0.20

0001-Fix-build-against-wayland-1.20.patch
removed since it is included in 2.0.20

License-Update: year updated to 2022.

Changelog:
=========
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.
Fixed hotplug controller detection, broken in 2.0.18

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>