st/wgl: improve selection of pixel format

st/wgl: improve selection of pixel format

Current selection of pixel format does not enforce the request of
stencil or depth buffer if the color depth is not the same as
requested.

For instance, GLUT requests a 32-bit color buffer with an 8-bit
stencil buffer, but because color buffers are only 24-bit, no
priority is given to creating a stencil buffer.

This patch gives more priority to the creation of requested buffers
and less priority to the difference in bit depth.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101703
Signed-off-by: Olivier Lauffenburger <o.lauffenburger@topsolid.com>
Tested-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>