temp/sdl2: fork from Alpine, upgrade to 2.0.15 (MR 1914)

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

temp/sdl2/khronos.patch

@@ -0,0 +1,18 @@
+From: sndirsch@suse.com
+Date: 2019-10-09 14:00:03+0000
+References: https://bugzilla.opensuse.org/show_bug.cgi?id=1153455
+
+--- a/include/SDL_opengl_glext.h
++++ b/include/SDL_opengl_glext.h
+@@ -472,8 +472,9 @@
+ typedef long GLsizeiptr;
+ typedef long GLintptr;
+ #else
+-typedef ptrdiff_t GLsizeiptr;
+-typedef ptrdiff_t GLintptr;
++#include <KHR/khrplatform.h>
++typedef khronos_ssize_t GLsizeiptr;
++typedef khronos_intptr_t GLintptr;
+ #endif
+ #define GL_BUFFER_SIZE                    0x8764
+ #define GL_BUFFER_USAGE                   0x8765