Fix up alpha buffer handling for Windows.

Fix up alpha buffer handling for Windows.
- add two new Pixel Format Descriptors that do not have alpha bits to
mirror the two that do.
- add logic to wglChoosePixelFormat to match PFD's with respect to alpha.
- Create/clear software alpha buffer as required.
Now a wgl or GLUT program can control the creation of a software alpha
buffer via the PFD or GLUT parms, respectively.