llvmpipe: more fixes for integer color buffers

llvmpipe: more fixes for integer color buffers

Cast back the fake floats to ints, and make sure we don't try to do scaling
in format conversion (which only makes sense with normalized values).
Also need to disable blending and alpha test (as per spec) for such buffers.
This makes fbo-blending from the piglit ext_texture_integer tests work for most
formats (some crash, and the luminance and intensity variants have the GB or
GBA channels respectively wrong).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>