Vertical text alignment fix

[acurrieclark]

An attempt has previously been made to fix this in #2938, but I don't think it worked as intended.

This fix has been working well for me an I hope it can help others.

[yorickshan]

thx for your pr, this fix has been merged to html2canvas-pro

[matortheeternal]

@yorickshan This change broke font rendering by html2canvas-pro when using a non-default line height.

Specifically: fontSize is not the pixel offset of the baseline of text when lineHeight is not = to the browser default of 1.175.

I have not verified that the old baseline returned by getFontMetrics returns the correct value, but I believe it is more logically correct than simply assuming fontSize is the distance from the top of the rect to the baseline, and a simple test can show this. Simply set line-height: 1 and you will observe a 17.5% difference in the position of text when comparing the browser to the canvas produced by html2canvas-pro.