Revert "glretrace: Don't request alpha bits pixel formats on Windows."

This reverts commit 6a8f8bd05043e7b8f8e26ad390c82021b995ee4d, as it
causes wrong rendering on traces that rely on alpha bits.

The proper solution is two-fold:
- replace use of ChoosePixelFormat with DescribePixelFormat or
  wglChoosePixelFormat
- match the visuals in the traces