[rcore] [SDL2] Add implementation for `FLAG_WINDOW_ALWAYS_RUN` (#4598)
* Enable FLAG_WINDOW_ALWAYS_RUN by default on PLATFORM_DESKTOP_GLFW
* Revert enabling FLAG_WINDOW_ALWAYS_RUN by default on PLATFORM_DESKTOP_GLFW
* Add implementation for FLAG_WINDOW_ALWAYS_RUN on PLATFORM_DESKTOP_SDL
* Add reset for GetFrameTime()