Render bars with a minimum of 1px width

[markov00]

There are cases where the computed bar width is too small due to a high number of data points or due to a misconfigured chart.
In this case, the chart can be rendered with a bar width smaller than 1 pixel, causing the canvas fillRect to paint nearly transparent bars due to the subpixel rendering.

I believe we can avoid that by always rendering a minimum of 1px wide bars, in these cases the bars can overlap but at least they are visible.

see elastic/kibana#184744