QuadVertices or verticesBackend.slice reused its backend slice.
This caused a race condition. QuadVertices can be accessed from
multiple goroutines, and resetting the head and copying the data
at internal/graphicscommand might not be synced.
This change fixes this issue by basically reverting
9cb631e30f.
Closes#1546
It looks like the allocation cost is pretty high even on Wasm.
Revive the special method not only on GopherJS but also on Wasm.
examples/sprites kept 30FPS without this fix, but keeps 35FPS with
this fix, on Hajime's MacBook Pro 2020 (macOS Catalina 10.15.6).
Updates #797
