Add support for fill-pattern

[songyumeng]

For #82

[ahocevar]

Thanks! This will even look nicer with openlayers/openlayers#8210.

[songyumeng]

It's cool.I found same problems about openlayers/openlayers#8210 .
I modify the ol.render.canvas.Replay.prototype.applyFill method in my project like this :

    ol.render.canvas.Replay.prototype.applyFill = function (state, geometry) {
        var fillStyle = state.fillStyle;
        var fillInstruction = [ol.render.canvas.Instruction.SET_FILL_STYLE, fillStyle];
        if (typeof fillStyle !== 'string') {
            var viewExtent = this.map.getView().getProjection().getExtent();
            fillInstruction.push([viewExtent[0], viewExtent[3]]);
        }
        this.instructions.push(fillInstruction);
    };

But this function must define after the ‘map‘ object is defined,and it is not a good solution.
Your PR for openlayers is very helpful to me.Thanks again.