mesa/readpixels: check strides are equal before skipping conversion

mesa/readpixels: check strides are equal before skipping conversion

The CTS packed_pixels test checks that readpixels doesn't write
into the space between rows, however we fail that here unless
we check the format and stride match.

This fixes all the core mesa problems with CTS packed_pixels
tests.

Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>