gallium: convert rasterizer state to use gl-style front/back concepts

gallium: convert rasterizer state to use gl-style front/back concepts

Use front/back instead of cw/ccw throughout.
Also, use offset_point/line/fill instead of offset_cw/ccw.

Brings gallium representation of this state into line with its main
user, and also what turns out to be the most common hardware
representation.

This fixes a long-standing bias in the interface towards the
architecture of the software rasterizer.