st/xa: Render update. Better support for solid pictures

st/xa: Render update. Better support for solid pictures

Remove unused and obsolete code for gradients and component-alpha
Support solid source- and mask pictures using a variable number
of samplers in the composite pipeline rather than the fixed number
we used before.

Tested using rendercheck for XA.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>