Added `graphics/SDL2/SpriteBatch` header
Fixed namespaces
Added empty class `SDL2::SpriteBatch < graphics::SpriteBatch`
`SDL2::SpriteBatch` now inherits `graphics::SpriteBatch` as public
In `SDL2::SpriteBatch`:
  - added `base` typedef;
  - added protected Drawer struct, which inherits from `base::Drawer`;
  - overrided `{begin, end}`
Added `graphics/SDL2/SpriteBatch` source file
In `SDL2::SpriteBatch`:
  refractored:
    - constructor
  added:
    - protected `renderer_handle` typedef
    - protected `_renderer_handle` field
Added defintion of `SDL2SpriteBatch(renderer_handle)` constructor
In `SpriteBatch`:
  in `begin()`:
    - sets background color;
    - clear everything (that makes background color visible);
  in `end()`:
    - presents the result of rendering
Added overrided `SDL2::SpriteBatch::backgroundColor` accessors
Added protected `SDL2::SpriteBatch::_bgcolor` field
Added definitions of `SpriteBatch::backgroundColor` accessors