virgl: Ignore empty transfer ranges
util_init_range initializes transfer ranges to have a start value of ~0 which translates to -1 and results in a negative offset for some cases. This in turn results in failing tests. When such a transfer range is encountered, then don't do the transfer at all.
Fixes: db77573d virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109190
Signed-off-by: Gert Wollny gert.wollny@collabora.com