Export image/svg sometime have big blank space on top and bottom

[nhymxu]

Describe the bug
Sometime, when I write very long sequence diagram, export image or svg always have big blank space on top/bottom image/svg.

I don't want this.

To Reproduce
Steps to reproduce the behavior:

1. Go to Bug link
2. Click on Actions at Left-side panel
3. Keep PNG size: auto (default)
4. Click download PNG button
5. Open image and see error

Expected behavior
Not have blank space on top/bottom of image/svg

Screenshots

Desktop (please complete the following information):

• OS: macOS 12.1
• Browser: Chrome
• Version: 96

Suggest solution
When using inspector I found that

SVG element on DOM have large height: 3637

So when export, export function using this height
https://github.com/mermaid-js/mermaid-live-editor/blob/18a83a807a6ace0cda4d6d3f6008b7c9d87ea3d0/src/lib/components/actions.svelte#L26-L29

If svg element not have this attribute => export function will re-calculate and using fitable height.

See, so new code maybe:

		const svg: HTMLElement = document.querySelector('#container svg');
		svg.removeAttribute('height');
		const box: DOMRect = svg.getBoundingClientRect();
		canvas.width = box.width;
		canvas.height = box.height;

[sidharthv96]

Had some discussions in mermaid-js/mermaid-live-editor#567