resnet-* example code should reuse Tensor memory for images

[nickgg]

In #2896 a version of readPngImage was added that loads into an existing Tensor. Convert the resnet example code (resnet-verify and resnet-runtime) to use this format to limit the total number of Tensors it allocates to some fixed (configurable) number. The more complex part here is how you rate limit the example code to wait until a Tensor is available.