Revert "Pass device filters by copy"
Revert "Pass device filters by copy"

This reverts commit 944fe3b2a4c223dc65e2c2fb6c18f605fba504b7.

The reverted commit was trying to solve issue with segfaulting
vex::Filter::General. As it happens, it is enough to pass filters by
value into vex::Filter::General constructor.

ddemidov/vexcl,Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,kapadia/vexcl,ddemidov/vexcl,Poulpy21/vexcl