Your card is actually really good for the time it was made/compared to some low end cards today. The problem is Blockland on Mac. I'm not sure if its a Mac problem or a Blockland problem but maybe both, I expierenced the same problem you do on a 2011 Mac book pro (AMD 6000 series graphics card) and I get the lines on everything also.
There's currently no way of fixing this, you can try maybe getting Badspot's attention by making a forum/poll and having people vote on a poll that want it fixed. If a lot of people vote Badspot will probably attempt to look into it, it's better than nothing.