Refactor Render Texture-Cache Interface

Since SDL2 is using in-memory bitmaps the same as Canvas engine, the
function names of the render.Engine interface have been cleaned up:

* NewTexture(filename, image) -> StoreTexture(name, image)
  Create a new cached texture with a given name.
* NewBitmap(filename) -> LoadTexture(name)
  Recall a stored texture with a given name.
* level.Chunk.ToBitmap uses simpler names for the textures instead of
  userdir.CacheFilename file-like paths.

image.go

@@ -55,7 +55,7 @@ func OpenImage(e render.Engine, filename string) (*Image, error) {
 		return nil, fmt.Errorf("OpenImage: %s: not a supported image type", filename)
 	}
 
-	tex, err := e.NewBitmap(filename)
+	tex, err := e.LoadTexture(filename)
 	if err != nil {
 		return nil, err
 	}