externals: Update SDL to 107db2d8

In light of 72a49c2bbc, the SDL submodule also needs updated. Updates
to the same commit used by the SDL package in ext-windows-bin.

CMakeLists.txt

@@ -292,7 +292,7 @@ if (ENABLE_SDL2)
         target_link_libraries(SDL2 INTERFACE "${SDL2_LIBRARY}")
         target_include_directories(SDL2 INTERFACE "${SDL2_INCLUDE_DIR}")
     else()
-        find_package(SDL2 2.0.14 QUIET)
+        find_package(SDL2 2.0.15 QUIET)
 
         if (SDL2_FOUND)
             # Some installations don't set SDL2_LIBRARIES
@@ -305,7 +305,7 @@ if (ENABLE_SDL2)
             add_library(SDL2 INTERFACE)
             target_link_libraries(SDL2 INTERFACE "${SDL2_LIBRARIES}")
         else()
-            message(STATUS "SDL2 2.0.14 or newer not found, falling back to externals.")
+            message(STATUS "SDL2 2.0.15 or newer not found, falling back to externals.")
         endif()
     endif()
 endif()