Factor out duplicate Inflater setup in WindowCursor

Since we use this code twice, pull it into a private method.  Let
the compiler/JIT worry about whether or not this logic should be
inlined into the call sites.

Change-Id: Ia44fb01e0328485bcdfd7af96835d62b227a0fb1
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>