Add --scale command line argument to mandelbrot and rt examples.

This applies a floating-point scale factor to the image resolution;
it's useful for experiments with many-core systems where the 
base image resolution may not give enough work for good load-balancing
with tasks.

examples/mandelbrot_tasks/mandelbrot.cpp

@@ -99,8 +99,12 @@ ensureTargetISAIsSupported() {
     }
 }
 
+static void usage() {
+    fprintf(stderr, "usage: mandelbrot [--scale=<factor]\n");
+    exit(1);
+}
 
-int main() {
+int main(int argc, char *argv[]) {
     unsigned int width = 1536;
     unsigned int height = 1024;
     float x0 = -2;
@@ -108,6 +112,25 @@ int main() {
     float y0 = -1;
     float y1 = 1;
 
+    if (argc == 1)
+        ;
+    else if (argc == 2) {
+        if (strncmp(argv[1], "--scale=", 8) == 0) {
+            float scale = atof(argv[1] + 8);
+            if (scale == 0.f)
+                usage();
+            width *= scale;
+            height *= scale;
+            // round up to multiples of 16
+            width = (width + 0xf) & ~0xf;
+            height = (height + 0xf) & ~0xf;
+        }
+        else 
+            usage();
+    }
+    else
+        usage();
+
     ensureTargetISAIsSupported();
 
     int maxIterations = 512;