New structure of Surface and Color, remove all internal SDL_Surface
Remove the colorkey notion. If we want to set a colorkey with SDL_Texture, it has to be done before convert the SDL_Surface to SDL_Texture. Idem for the pixel mask of PixelBits.cpp
Replace SDL_Surface by SDL_Texture except in the Color class. Build but not run.
Fix subsurface storing
Forgot to replace the clipping rectangle
Fix Surface::raw_draw_region
Fix possible segfault
Merge branch master
Remove intermediate texture again after merge conflict
Fix comments
Add alpha channel to Color
MapEntities::build_non_animated_tiles : use transparent background again
Add alpha channel handling for Color on Lua side
Allow the alpha channel to be nil on Lua side
Merge branch 'master' into texture

Conflicts:
	include/lowlevel/Color.h
	include/lowlevel/VideoManager.h
	src/lowlevel/VideoManager.cpp
Fill the Surface with transparent color if created without image
Handle merge conflict
Allow a surface to not have an internal texture, and create it only if we have to
Update comment block after merge conflict
Merge conflict