snapshot: Avoid triggering assertion for grabbing the target during d…

…estroy

If the source wins the race to acquire the original surface as it is
being destroyed, it triggers an assertion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-surface-inline.h

@@ -49,4 +49,12 @@ _cairo_surface_flush (cairo_surface_t *surface)
     return status;
 }
 
+static inline cairo_surface_t *
+_cairo_surface_reference (cairo_surface_t *surface)
+{
+    if (!CAIRO_REFERENCE_COUNT_IS_INVALID (&surface->ref_count))
+	_cairo_reference_count_inc (&surface->ref_count);
+    return surface;
+}
+
 #endif /* CAIRO_SURFACE_INLINE_H */

src/cairo-surface-snapshot-inline.h

@@ -37,6 +37,7 @@
 #define CAIRO_SURFACE_SNAPSHOT_INLINE_H
 
 #include "cairo-surface-snapshot-private.h"
+#include "cairo-surface-inline.h"
 
 static inline cairo_bool_t
 _cairo_surface_snapshot_is_reused (cairo_surface_t *surface)
@@ -51,7 +52,7 @@ _cairo_surface_snapshot_get_target (cairo_surface_t *surface)
     cairo_surface_t *target;
 
     CAIRO_MUTEX_LOCK (snapshot->mutex);
-    target = cairo_surface_reference (snapshot->target);
+    target = _cairo_surface_reference (snapshot->target);
     CAIRO_MUTEX_UNLOCK (snapshot->mutex);
 
     return target;