More progress on structures. Modify structure template.

Notice 'a' used for .TS/.TE will has effect on other columns, so used
once for it to have a space for its width then other columns could be
defined with just 'l'.

COVER.md
src/SDL_AudioCVT.3
src/SDL_AudioDeviceEvent.3
src/SDL_AudioSpec.3
src/SDL_AudioStream.3
src/SDL_Color.3
src/SDL_ControllerAxisEvent.3
src/SDL_ControllerButtonEvent.3
src/SDL_ControllerDeviceEvent.3
src/SDL_DisplayMode.3
src/SDL_DollarGestureEvent.3
src/SDL_StructureTemplate