Texture refactoring (mega-commit)

 * Added all available texture targets (as of OpenGL 4.2)
 * Added possibility to load every texture target that supports
   it from source data
 * Added -gnatwa switch for compiling, fixed warnings that got
   raised by it
 * Added getters backed by glGetTexLevelParameter*
 * Removed old GL.Objects.Textures.Loader_2D; functionality is
   now available with the new texture targets.