Use C++ style comments consistently in SDL source code

Implemented using this script:

find . -type f -exec sed -i'' -e 's,/\* *\([^*]*\)\*/ *$,// \1,' -e 's, \+$,,' {} \;
git checkout \
    core/linux/SDL_evdev_kbd_default_keymap.h \
    events/imKStoUCS.* \
    hidapi \
    joystick/controller_type.c \
    joystick/controller_type.h \
    joystick/hidapi/steam/controller_constants.h \
    joystick/hidapi/steam/controller_structs.h \
    joystick/SDL_gamepad_db.h \
    libm \
    render/*/*Shader*.h \
    render/vitagxm/SDL_render_vita_gxm_shaders.h \
    render/metal/SDL_shaders_metal_*.h \
    stdlib/SDL_malloc.c \
    stdlib/SDL_qsort.c \
    stdlib/SDL_strtokr.c \
    test/ \
    video/directx/SDL_d3d12_xbox_cmacros.h \
    video/directx/d3d12.h \
    video/directx/d3d12sdklayers.h \
    video/khronos \
    video/x11/edid-parse.c \
    video/x11/xsettings-client.* \
    video/yuv2rgb
sed -i'' -e 's,/\* *\([^*]*\)\*/ *$,// \1,' -e 's, \+$,,' hidapi/SDL_hidapi.c

src/SDL_properties.c

@@ -177,7 +177,7 @@ SDL_PropertiesID SDL_CreateProperties(void)
     SDL_UnlockMutex(SDL_properties_lock);
 
     if (inserted) {
-        /* All done! */
+        // All done!
         return props;
     }
 
@@ -225,7 +225,7 @@ int SDL_CopyProperties(SDL_PropertiesID src, SDL_PropertiesID dst)
             SDL_Property *dst_property;
 
             if (src_property->cleanup) {
-                /* Can't copy properties with cleanup functions, we don't know how to duplicate the data */
+                // Can't copy properties with cleanup functions, we don't know how to duplicate the data
                 continue;
             }