WebGL2 backend doesn't update uniform buffers correctly

[andrewvarga]

Description
The WebGL backend doesn't seem to update uniform buffers correctly.

Repro steps
All details are in this thread by @zicklag
#3003

Expected vs observed behavior
See above

Extra materials

Platform
This is happening on the WebGL 2 backend regardless of OS / browser vendor.
Tried: Chrome, Firefox, Mac, Windows.