Updating sprite image is not reflected in currently rendered layers

[ivovandongen]

In #6375 an api is added to add and remove Sprite images. While implementing the bindings for Android (#6513) I noticed that updating a sprite doesn't affect anything visually.

Initial situation

• Add a sprite image
• Add a geojson source with a point
• Add a symbol layer with that source and source-image set to the sprite image identifier

Sprite is displayed correctly,

Updating the sprite by adding a new image with the same identifier
Image is not updated

As the size of the sprite cannot be changed, a log entry is shown, nothing more. This might be confusing for developers. See: https://github.com/mapbox/mapbox-gl-native/blob/master/src/mbgl/sprite/sprite_atlas.cpp#L132

Update the sprite by first removing and then adding a sprite with the same identifier
Image is not updated

In this case; no log entry is shown, nothing happens visually.

cc @tmpsantos

[jfirebaugh]

Updating the sprite by adding a new image with the same identifier

I don't think we should allow this. If we want an update API, let's make it an explicit updateImage method.

[1ec5]

Related: #6536.