add skpdiff tool to compare bitmaps

experimental/skpdiff/diff_pixels.cl

+#pragma OPENCL_EXTENSION cl_khr_global_int32_base_atomics
+
+const sampler_t gInSampler = CLK_NORMALIZED_COORDS_FALSE |
+                             CLK_ADDRESS_CLAMP_TO_EDGE   |
+                             CLK_FILTER_NEAREST;
+
+__kernel void diff(read_only image2d_t baseline, read_only image2d_t test, __global int* result) {
+    int2 coord = (int2)(get_global_id(0), get_global_id(1));
+    uint4 baselinePixel = read_imageui(baseline, gInSampler, coord);
+    uint4 testPixel = read_imageui(test, gInSampler, coord);
+    int4 pixelCompare = baselinePixel == testPixel;
+    if (baselinePixel.x != testPixel.x ||
+        baselinePixel.y != testPixel.y ||
+        baselinePixel.z != testPixel.z ||
+        baselinePixel.w != testPixel.w) {
+
+        atom_inc(result);
+    }
+}