Xpra: Ticket #250: review and fix/disable strict aliasing GCC warnings

Recent versions of GCC will print lots of scary warnings when building some of the Cython code:

• wimpiggy.lowlevel:

  wimpiggy/lowlevel/bindings.c: In function '__pyx_pf_8wimpiggy_8lowlevel_8bindings_44is_mapped':
  wimpiggy/lowlevel/bindings.c:7248:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  wimpiggy/lowlevel/bindings.c:7248:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  (...)
  

And another dozen after that..

• rencode:

  xpra/rencode/rencode.c: In function '__pyx_f_4xpra_7rencode_8_rencode_decode':
  xpra/rencode/rencode.c:5540:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:5540:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:5576:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:5576:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c: In function 'init_rencode':
  xpra/rencode/rencode.c:6027:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:6027:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:6071:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  xpra/rencode/rencode.c:6071:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  

we should fix the warnings, preferably by making the compiler happy or if needed by adding f[no-]strict-aliasing where needed.