svga: implement MSAA alpha_to_one feature

svga: implement MSAA alpha_to_one feature

The device doesn't directly support this feature so we implement it with
additional shader code which sets the color output(s) w component to
1.0 (or max_int or max_uint).

Fixes 16 Piglit ext_framebuffer_multisample/*alpha-to-one* tests.

v2: only support unorm/float buffers, not int/uint, per Roland.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>