gallium: add support for layered rendering

gallium: add support for layered rendering

Since pipe_surface already has all the necessary fields no interface
changes are necessary except adding a new shader semantic value
(TGSI_SEMANTIC_LAYER).
(Note that what GL knows as "gl_Layer" variable d3d10 is naming
"RENDER_TARGET_ARRAY_INDEX".)

v2: drop cap bit (just tied to geometry shader), add docs.