feat: support round cap for polar bars

[Ovilia]

Fix #4856 .

Set roundCap to be true (default false) for polar bars will have this effect.

var option = {
    angleAxis: {
        max: 5
    },
    radiusAxis: {
        type: 'category',
        data: ['a', 'b', 'c'],
        z: 10
    },
    polar: {
    },
    series: [{
        type: 'bar',
        data: [1, 2, 3],
        coordinateSystem: 'polar',
        name: 'A',
        roundCap: true,
        color: 'rgba(200, 0, 0, 0.5)',
        itemStyle: {
            borderColor: 'red',
            borderWidth: 1
        }
    }],
    legend: {
        show: true,
        data: ['A']
    },
    tooltip: {
        show: true
    }
};

It may also help to provide an array as polar.radius to make rings more easily. Currently, this can be done with empty data in the inner side:

This will be improved in future PRs. I'll see if I can make it in this milestone. Please ignore this feature for now.

TODO:

• feat: support polar.radius
• feat: support bar background

[Ovilia]

Polar radius supports array now. b5e6ba7

[maplness]

Is there an offset property that allows those bars don't need to start from 0 point for all?

[refanbanzhang]

nice work!

[iaminvictus1993]

mark,bar stack 做环形图倒角