llvmpipe: support rendering to buffer render targets.

llvmpipe: support rendering to buffer render targets.

Unfortunately not usable from OpenGL, and no cap bit.
Pretty similar to a 1d texture, though allows specifying a start element.

v2: also fix up renderbuffer width (which will get promoted to fb width)
to be the number of elements

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