Platform: clarify CMake subproject setup for Sdl2Application.

Spent several hours trying to figure out why magnum-player built against
static SDL (which uses the flags documented here) was hellishly laggy.
Didn't expect it was due to timers missing. OTOH, why SDL does even
allow itself to be built in such an unusable state?

src/Magnum/Platform/Sdl2Application.h

@@ -227,14 +227,14 @@ necessary.
 
 @code{.cmake}
 # This is the most minimal set of features which still make Sdl2Application
-# work. If you need something from these, remove the setting. The SDL_AUDIO
-# option should not be needed either as Magnum doesn't use it, but if it's
-# disabled it causes linker errors. Either SDL_DLOPEN or SDL_LOADSO needs to be
-# enabled depending on the system to allow linking dependencies at runtime, so
-# it's better to just leave them both on.
+# work. If you need something from these, remove the setting. The SDL_AUDIO and
+# SDL_EVENT options should not be needed either as Magnum doesn't use them, but
+# if they're disabled they causes compiler or linker errors. Either SDL_DLOPEN
+# or SDL_LOADSO needs to be enabled depending on the system to allow linking
+# dependencies at runtime, so it's better to just leave them both on. The
+# SDL_TIMERS option is important for rendering performance.
 set(SDL_ATOMIC OFF CACHE BOOL "" FORCE)
 set(SDL_CPUINFO OFF CACHE BOOL "" FORCE)
-set(SDL_EVENTS OFF CACHE BOOL "" FORCE)
 set(SDL_FILE OFF CACHE BOOL "" FORCE)
 set(SDL_FILESYSTEM OFF CACHE BOOL "" FORCE)
 set(SDL_HAPTIC OFF CACHE BOOL "" FORCE)
@@ -243,7 +243,6 @@ set(SDL_POWER OFF CACHE BOOL "" FORCE)
 set(SDL_RENDER OFF CACHE BOOL "" FORCE)
 set(SDL_SENSOR OFF CACHE BOOL "" FORCE)
 set(SDL_THREADS OFF CACHE BOOL "" FORCE)
-set(SDL_TIMERS OFF CACHE BOOL "" FORCE)
 # This assumes you want to have SDL as a static library. If not, set SDL_STATIC
 # to OFF instead.
 set(SDL_SHARED OFF CACHE BOOL "" FORCE)