gallium: Replace gl_rasterization_rules with lower_left_origin and half_pixel_center.

gallium: Replace gl_rasterization_rules with lower_left_origin and half_pixel_center.

Squashed commit of the following:

commit 04c5fa2cbb8e89d6f2fa5a75af1cca03b1f6b852
Author: José Fonseca <jfonseca@vmware.com>
Date:   Tue Apr 23 17:37:18 2013 +0100

    gallium: s/lower_left_origin/bottom_edge_rule/

commit 4dff4f64fa83b9737def136fffd161d55e4f1722
Author: José Fonseca <jfonseca@vmware.com>
Date:   Tue Apr 23 17:35:04 2013 +0100

    gallium: Move diagram to docs.

commit 442a63012c8c3c3797f45e03f2ca20ad5f399832
Author: James Benton <jbenton@vmware.com>
Date:   Fri May 11 17:50:55 2012 +0100

    gallium: Replace gl_rasterization_rules with lower_left_origin and half_pixel_center.

    This change is necessary to achieve correct results when using OpenGL
    FBOs.

Reviewed-by: Marek Olšák <maraeo@gmail.com>