Fixed blur filter throwing exception for too large values relative to image

[Rishab87]

In this PR I've clamped radius to ensure we calculate maximum allowed blur in case of too large values relative to the image
I've also added a check for sum to make it 1 if it is 0 because it can be still zero in some cases even after clamping.

I tested it for the code provided in the issue and it works without crashing:

void setup() {
  size(100, 100, P3D);
}

void draw() {
  background(0);
  ellipse(50, 50, 80, 80);
  filter(BLUR, 29);
}

Output:

Their can be other approaches too, like throwing a meaningful error for the users.

This PR fixes #723

[Stefterv]

Hi @Rishab87 thank you for your work on this, just a quick question, you are clamping the input value as well as doing a safer divide? Could you explain why do both?

[Rishab87]

even after clamping I think their can be few edge cases like when loop would iterate a few times where sum can accumulate to zero though I'm not completely sure about this to be honest