libdeng2|Record: Added an explicit setter for char const*

Evidently some compilers (MSVC at least) interpret a zero-length,
c-style string as bool, consequently resulting in a NumberValue
rather than a TextValue being written to the Record.
Windows|Debug|libgui: Don't assert GL operation success

At shutdown, after restoring the original display mode, there was an
OpenGL error occurring that caused a crash (assert dialog => recursive
repaint?).