adds a method that only redraw the tiles

this commit [1] is backwards incompatible since the previous version didn't reload the tiles on redraw. 

I think with this layer there is a naming issue, redraw using canvas sounds like "redraw the tiles in the canvas" but in a tiled layer redraw means reload the tiles.

My proposal here is to add a new method called redrawTiles that updates the content on the screen without loading the tiles. This also uses requestAnimationFrame to render the canvas tiles.

I don't know it drawTiles is the best name since it can be confused with the redraw method

[1] https://github.com/Leaflet/Leaflet/commit/724f9aa3d8071a9c409987aa0f1dd059d2ec47bc