intel: Add support for ARB_texture_float.

intel: Add support for ARB_texture_float.

For 1 and 2-channel formats the hardware only supports rendering to R
and RG.  To do I and L render targets we just call them R and
everything works out.  For A, we would need to rewrite the CC to do
the alpha channel's blending on color instead, and send the fragment
alpha down the red channel.  For LA, there doesn't seem to be any
hope, because we can't do independent color/alpha blending while
treating the LA surface as RG.

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