st/mesa: overhaul vertex setup for clearing, glDrawPixels, glBitmap

st/mesa: overhaul vertex setup for clearing, glDrawPixels, glBitmap

Define a new st_util_vertex structure which is a bit smaller (9 floats
versus the previous 12 floats per vertex).  Clean up the glClear,
glDrawPixels and glBitmap code that sets up the vertex data and does the
drawing so it's all very similar.  This can lead to more consolidation.

v2: add assertion that vertex buffer slot == 0 to catch possible future
change in cso_get_aux_vertex_buffer_slot() behavior.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>