gallium: Add support for multiple viewports

Gallium supported only a single viewport/scissor combination. This
commit changes the interface to allow us to add support for multiple
viewports/scissors.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: José Fonseca<jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
52 files changed