Add checks for loops being created on nodes

[fnRaihanKibria]

Addressing issue #1932

The cause of the crash is a buffer overrun in ShaderGraph::topologicalSort() in the line _nodeOrder[count++] = node;. This is caused by a loop edge being present on the 'color_mix' node of the graph object this was called on, which made the sort algorithm malfunction.

Unfortunately I was unable to determine in the time I have available why this loop is created. I didn't want to add a change that hides the underlying problem, which still needs to be fixed. Instead, this PR adds some checks to two locations in the code that throw exceptions when a case of a loop being created is detected, to aid in fixing the real issue and maybe help diagnosing other problems in the future. With this PR the graph editor does not crash any more, instead an error message "Upstream node 'color_mix' has itself as downstream node, creating a loop" is printed to console.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fnRaihanKibria (e58370d)

[jstone-lucasfilm]

This looks like a great first step, thanks @fnRaihanKibria! I'm CC'ing @niklasharrysson for his expertise with this code, and we can discuss next steps.