mesa: add gl_driver_flags::NewScissor{Rect,Test}

mesa: add gl_driver_flags::NewScissor{Rect,Test}

_NEW_SCISSOR mesa flag is set when a scissor test is enabled/disabled
or when a new rectangle is defined. However, it triggers too much
changes in the state tracker.

Actually, ST_NEW_RASTERIZER should only be called when a scissor
test is enabled/disabled, while ST_NEW_SCISSOR should be called
in both situations.

In other words, this will avoid to update the rasterizer every
time a new rectangle is defined using glScissor*().

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>