Update formatting.
Refactor the ttexture image loading constructor.

The image to surface conversion part has been moved to its own helper
function. This will be used for a constructor using a SDL_Surface as
base.
Add a texture constructor taking a SDL_Surface.
Add a texture constructor taking a surface.
Add a draw function for the window.
Add a clear function to the window.

Also clear the window in the constructor.
Adds a basic version of the SDL2 test tool.

The tool is used to test parts of the new SDL2 code, without the need
to directly modify the main Wesnoth code.
Implement the window command.
Merge branch 'SDL2_test_tool'

The branch contains several improvements for the SDL2 code and adds a
small tool to test the code outside of the main code.