perf: Only print description once per backend

Currently we print the backend description before every time, which is
overly verbose. As the information doesn't^Wshouldn't change, simply
print it before running the first test of each target.

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

perf/cairo-perf-trace.c

@@ -278,6 +278,10 @@ describe (cairo_perf_t *perf,
 {
     char *description = NULL;
 
+    if (perf->has_described_backend)
+	    return;
+    perf->has_described_backend = TRUE;
+
     if (perf->target->describe)
         description = perf->target->describe (closure);
 
@@ -827,6 +831,7 @@ main (int   argc,
 
 	perf.target = target;
 	perf.test_number = 0;
+	perf.has_described_backend = FALSE;
 
 	if (perf.exact_names) {
 	    for (n = 0; n < perf.num_names; n++) {

perf/cairo-perf.h

@@ -91,6 +91,7 @@ typedef struct _cairo_perf {
     const cairo_boilerplate_target_t **targets;
     int num_targets;
     const cairo_boilerplate_target_t *target;
+    cairo_bool_t has_described_backend;
     unsigned int test_number;
     unsigned int size;
     cairo_t *cr;