Prevent access violation in motioner's preview window rendering

Mesh::decalDraw() have not considered the case glClientActiveTextureARB
is not loaded.  Modern GPUs should have it, but motioner project
intentionally skipped loading the function pointer via wglGetProcAddress().

Note that motioner project won't immediately crash by the error since
access violation exception is handled.  The end result looks like
the preview window show nothing after loading some model.