Unset fill-outine-color duplicates fill-color paint buffer if the latter is data-driven

[mourner]

When a fill layer has a data-driven fill-color but fill-outline-color is unset (which is very common in our styles), it inherits the value and we end up uploading two identical, independently generated paint buffers, increasing the GPU memory footprint of rendering such layers:

mapbox-gl-js/src/style/style_layer/fill_style_layer.js

Lines 53 to 56 in 4eeefae

	const outlineColor = this.paint._values['fill-outline-color'];
	if (outlineColor.value.kind === 'constant' && outlineColor.value.value === undefined) {
	this.paint._values['fill-outline-color'] = this.paint._values['fill-color'];
	}

We should optimize this somehow — maybe add special code to reuse the fill-color paint buffer for the outline if the value is undefined?