Fix for compiler errors in Visual Studio 2008.

Bug:
- Visual Studio 2008 errors in Gwen::Min about Extension.size() returning
  an ambiguous value (int vs. unsigned).

Fix:
- Replaced the hardcoded 512 with sizeof(FilterBuffer) since
  Extension.size()and sizeof() both return a size_t.

Misc Changes:
- Changed some hard-coded values used multiple times into a defined value.
  Makes the code more readable and less prone to errors if the values need
  to change.