Safari doesn't render transparent gradients well

[thisandagain]

Expected Behavior

Please describe what should happen
This project https://preview.scratch.mit.eduLLK/scratch-gui#222183421 should look like:

Actual Behavior

Describe what actually happens
On Safari:

Steps to Reproduce

Explain what someone needs to do in order to see what's described in Actual behavior above
View https://preview.scratch.mit.edu/#222183421

Operating System and Browser

e.g. Mac OS 10.13.5 Safari 11.1.1

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Moved from scratchfoundation/scratch-gui#2654
/cc @chrisgarrity