GCC 4.5 compatibility: ICE and miscompiled binary operators.

It seems that in this particular case `a &= b` is not doing the same as
`a = a & b`. In Release build the expression is miscompiled (it
always resets `a` to zero), in Debug build it triggers ICE:

    Object.hpp:280:9: internal compiler error: in make_decl_rtl, at varasm.c:1318

Possibly related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43880.
Changed to `a = a & b` in all cases, now SceneGraphObject test passes
again. Outside of Object (e.g. in Corrade's EnumSet tests) this is not
reproducible, wtf.

Moreover, classic solution, `-fno-strict-aliasing` didn't help at all
here. Probably caused by some other optimization, IMHO.