+some uniform related improvements

examples_ptx/mandelbrot_tasks/mandelbrot_tasks.ispc

@@ -82,8 +82,8 @@ mandelbrot_ispc(uniform float x0, uniform float y0,
                 uniform float x1, uniform float y1,
                 uniform int width, uniform int height, 
                 uniform int maxIterations, uniform int output[]) {
-    uniform float dx = (x1 - x0) / width;
-    uniform float dy = (y1 - y0) / height;
+    const uniform float dx = (x1 - x0) / width;
+    const uniform float dy = (y1 - y0) / height;
     const uniform int xspan = max(32, programCount*2);  /* make sure it is big enough to avoid false-sharing */
     const uniform int yspan = 16;