gallium: move clear paths from rgba to a pointer to a color union (v2)

gallium: move clear paths from rgba to a pointer to a color union (v2)

This moves the gallium interface for clears from using a pointer to 4 floats to a pointer to a union of float/unsigned/int values.

Notes:
1. the value is opaque.
2. only when the value is used should it be interpretered according to
the surface format it is going to be used with.
3. float clears on integer buffers and vice-versa are undefined.

v2: fixed up vega and graw, dropped hunks that shouldn't have been in
patch.

Signed-off-by: Dave Airlie <airlied@redhat.com>