RG8UNorm texture says it doesn't support random writes

Steps to reproduce:

1. Open the attached project and build for web
2.  
3.  

Actual results: 
Render is black and console errors about RG8Unorm not supporting random writes

Expected results: 

Reproducible with versions: 

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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