_text_sdl2.pyx: Don't clear pixel memory twice
_text_sdl2.pyx: Don't clear pixel memory twice

[SDL2 does this already](https://github.com/SDL-mirror/SDL/blob/release-2.0.0/src/video/SDL_surface.c#L97-L98)
rnixx/kivy,rnixx/kivy,jegger/kivy,matham/kivy,matham/kivy,matham/kivy,bionoid/kivy,jegger/kivy,matham/kivy,akshayaurora/kivy,kivy/kivy,KeyWeeUsr/kivy,kivy/kivy,jegger/kivy,KeyWeeUsr/kivy,KeyWeeUsr/kivy,LogicalDash/kivy,akshayaurora/kivy,inclement/kivy,Cheaterman/kivy,inclement/kivy,kivy/kivy,inclement/kivy,bionoid/kivy,rnixx/kivy,bionoid/kivy,LogicalDash/kivy,Cheaterman/kivy,KeyWeeUsr/kivy,akshayaurora/kivy,Cheaterman/kivy,jegger/kivy,bionoid/kivy,LogicalDash/kivy,LogicalDash/kivy,Cheaterman/kivy