Fixes glfwGetProcAddress calls that were causing a ReferenceError exception, trying to call unexisting function _getProcAddress.
On calls to glfwSetWindowSizeCallback, calls the provided callback function, as described by glfw documentation: When a callback function is set, it will be called with the current window size before this function returns.
Adds unit test for glfwGetProcAddress.
Merge pull request #2403 from guillaumeblanc/incoming

Fixes glfwGetProcAddress calls that were causing a ReferenceError except...