mesa: reduce the size of gl_viewport_attrib

mesa: reduce the size of gl_viewport_attrib

All drivers convert these to float, so there is no reason to use double.
The piglit test that expects double precision from glGet will be adjusted
not to require it (there is a piglit patch).

gl_context::ViewportArray: 512 -> 384 bytes

Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>