Optimize CustomMarkerClass draw Method

[radiantstatic]

Context on the issue: #90 and #162

Changes:

Forced redraw is a result of the following causing significant lag while zooming:

const height = this.element.offsetHeight;
const width = this.element.offsetWidth;

Instead of getting the width and height of a given element, we can use CSS transforms to position marker based on its already calculated width. When using position: absolute; top: x, left: x - we can also assume that the item is being positioned in the bottom right corner of a given marker (thus making repositioning easier).

If we want to continue to use the offsetX and offsetY optional configuration, just add it to point.x and point.y instead.

Result:
I've only been able to test locally, but I more or less have completely removed the lag associated with zooming on 500+ CustomMarkers. Looking for someone to help me validate.

[radiantstatic]

@HusamElbashir - Is this something you could take a look at? I know a lot of consumers of this would benefit greatly. Thanks!

[HusamElbashir]

Hey @radiantstatic thanks for the contribution. This definitely looks useful. Apologies though I just have so much going on at work and in my personal life atm that I didn't get a chance to look at it. Hoping to get to it soon though

[radiantstatic]

@HusamElbashir - Just wanted to follow up, trying to make a decision as to whether or not to fork this project. If you have some time to review over the next few days, let me know!