gallium/u_blitter: implement shader-based MSAA resolve with bilinear filtering

gallium/u_blitter: implement shader-based MSAA resolve with bilinear filtering

For scaled resolve. The filter is only good for magnification.

If somebody has an idea how to implement a good filter for minification,
I'm all ears. I'd have to use derivatives probably.

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