DXX-Rebirth v0.60 Beta 2

Fix -Wunused-variable warning in PNG screenshot code

Global `Viewer` was copied to local `viewer`, and the local should have
been used in this block.  Instead, the global was incorrectly used.
gcc-5 warns for this unused variable.  gcc-6 and gcc-7 incorrectly do
not warn.

Reported-by: Jayman2000 <https://github.com/dxx-rebirth/dxx-rebirth/issues/375>
Fixes: 131c1b9f4d556fbd9d67815755608602b408698b ("Add support for PNG screenshots")

DXX-Rebirth v0.59.100

DXX-Rebirth v0.59.100 (v0.60 BETA 1)