Use at least 16bit for Depth-Buffer (fixes icculus#37)

Seems like on Windows 0 bits is handled as "Let Windows/Driver decide),
so handle that case by setting a reasonable value of 16.

Sources/Engine/Graphics/SDL/SDLOpenGL.cpp

@@ -95,11 +95,7 @@ BOOL CGfxLibrary::SetupPixelFormat_OGL( HDC hdc, BOOL bReport/*=FALSE*/)
   // clamp depth/stencil values
   extern INDEX gap_iDepthBits;
   extern INDEX gap_iStencilBits;
-       if( gap_iDepthBits <12) {
-         ASSERT(gap_iDepthBits >= 12 && "Using gap_iDepthBits=0, that's probably bad?!");
-         gap_iDepthBits   = 0;
-       }
-  else if( gap_iDepthBits <22) gap_iDepthBits   = 16;
+       if( gap_iDepthBits <22) gap_iDepthBits   = 16; // this includes 0; 16 is a safe default
   else if( gap_iDepthBits <28) gap_iDepthBits   = 24;
   else                         gap_iDepthBits   = 32;
        if( gap_iStencilBits<3) gap_iStencilBits = 0;