draw-vertices-2101010: add half-scale color tests

Picks up the driver not bothering to normalize attribute values.
This was broken in an early version of my i965 support for this
extension.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
draw-vertices-2101010: add 'glsl' option.

At least i965 takes a fairly different path for FF and ARB VP
compared to GLSL vertex shaders. Add a 'glsl' option to allow
this test to exercise either path.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>