temp/sdl2: fork from Alpine, upgrade to 2.0.15

2.0.15 is a development 'version' (it'll never be released), and will be replaced by 2.0.16 when that is released. The reason for forking this is because there are some issues that are resolved here that prevent SDL2 from working on the Librem 5. The fixes cannot be easily backported to 2.0.14, hence the fork and upgrade.

fixes #950 (closed)