mesa 3.0 Win32 bugs

[Xiaoming Zhou]

I built mesa3.0 Win32 on Windows98 with VC++6.0 and
found some problems.

1. When call wglDeleteContext, which calls Dither function,
in Dither function the Current pointer to NULL. I just added
if(Current == NULL) return;   to make it work.

2. ChoosePixelFormat return 0 when PFD_DRAW_TO_BITMAP is used.
Just try Microsoft example glbmp.exe.