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>

src/mesa/main/readpix.c

@@ -523,7 +523,8 @@ read_rgba_pixels( struct gl_context *ctx,
        * convert to, then we can convert directly into the dst buffer and avoid
        * the final conversion/copy from the rgba buffer to the dst buffer.
        */
-      if (dst_format == rgba_format) {
+      if (dst_format == rgba_format &&
+          dst_stride == rgba_stride) {
          need_convert = false;
          rgba = dst;
       } else {