Upgrade to SDL2.0.20

[roncapat]

Fix #267

[roncapat]

@clalancette I noticed a problem with the sdl2_vendor/sdl2-windows-add-vcruntime.patch patch not being applied after switching to point release 20. I'll do some checks and report.

[roncapat]

Should be fixed now. I warn you that it seems to me that during CI the SDL doesn't get built from sources (it may already be present in Ubuntu packages). Maybe FORCE_BUILD_VENDOR_PKG is not set?

[clalancette]

Maybe FORCE_BUILD_VENDOR_PKG is not set?

Yeah, it is not: https://github.com/ros-drivers/joystick_drivers/blob/ros2/.github/workflows/rolling-build-ci.yaml

The vendoring of this package was mostly meant for Windows; on Linux, we generally have a packaged SDL available (though I guess in this case, it might be too old). One thing we could consider would be to build it if the SDL version is not new enough to have the game controller interface.

[clalancette]

Looks good to me, thanks for the update.