Add internal SDL_UpdateTextureFromSurface()

[1bsyl]

Add internal SDL_UpdateTextureFromSurface().
Since, SDL_CreateTextureFromSurface() is a big function, this makes it more readable.
Currently this only splits and lets the function internal.

Also: maybe we want a future SDL_CreateStreamingTextureFromSurface() or other variants for creating textures.
So maybe something like: SDL_CreateTextureWithPropertiesFromSurface() ?
so I guess, this would be a first step to prepare this.

in SDL_UpdateTextureFromSurface():

• it can resolve the ColorKey if needed. (was there)
• update texture alpha modulation and turn off/on blending if needed (was there)
• maybe some stuff from colorspace (was there)
• could handle streaming/static texture (not)