Fix transitions from property default values

[jfirebaugh]

Noticed while working on #2762.

map.setPaintProperty("circle", "circle-color", "red") when called on this layer will interpolate black to red correctly:

                {
                    "id": "circle",
                    "type": "circle",
                    "source": "geojson",
                    "paint": {
                        "circle-color": "black"
                    }
                }

However, with this layer it'll just jump from black to red without a transition:

                {
                    "id": "circle",
                    "type": "circle",
                    "source": "geojson",
                    "paint": {}
                }

The expected behavior is for both to have an interpolated transition.

[stepankuzmin]

Hi! I've done a little research and found, that StyleLayer#_paintDeclarations is empty for layers with default paint properties, while layers with specified paint properties have _paintDeclarations.

I've tried to set paint declaration to default after https://github.com/mapbox/mapbox-gl-js/blob/master/js/style/style_layer.js#L111 like this:

this._paintDeclarations[klass || ''][name] = new StyleDeclaration(this._paintSpecifications[name]);

but it does not affect transition. So am I missing something? Should we explicitly define default paint properties in StyleLayer constructor? (something like this)

for (paintName in this._paintSpecifications) {
    this.setPaintProperty(paintName, this._paintSpecifications[paintName].default);
}

[mourner]

@stepankuzmin I would suggest exploring style_transition.js for why it doesn't transition from a default property. There's an early bailout there somewhere probably.

[stepankuzmin]

It seems that there is no transition from defaults because there is no oldTransition to transition from. oldTransition for StyleLayer is set in _applyPaintDeclaration, which is called from updatePaintTransitions if StyleLayer._paintDeclarations is not empty. For layer with default paint properties, there is no _paintDeclarations, so there is no oldTransition also.

Defining default paint properties in StyleLayer constructor solves the problem, but I am not sure about this.