Allow bypass on ARM with MUTTER_ALLOW_BYPASS_COMPOSITOR envvar

We want to disable bypassing the compositor by default on ARM for now.
However, it's also useful to allow the normal operation to occur for
testing. To support that, check for the MUTTER_ALLOW_BYPASS_COMPOSITOR
environment variable.

[endlessm/eos-shell#2548]
Revert "window-actor: Disable _NET_WM_BYPASS_COMPOSITOR unredirection on ARM"

This reverts commit e0d5efcb152a6a18dbe478b557d1a98fabd3a4a2. It's now
disabled at the location where the actual atom is checked with an
associated envvar to allow normal operation.

[endlessm/eos-shell#2548]
Merge pull request #5 from endlessm/shell/2548

Better hack to disable compositor bypass on ARM