[sldev] Quirk in LIBPNG project files

[Nicholaz Beresford]

LIBPNG has a quirk in their VC71 project files (at least
in relation to how SL does things).

In the builds for LIB Debug and LIB Release (and possibly
also LIB ASM Debug and LIB Asm Release)

Properties, C++, Code Generation is set to link against the
DLL Runtime Libraries.   This causes a series of warnings
when linking libpng to the SL viewer.

For SL the settings there should be /MT and MTd rather than
/MD and /MDd



Nick