vtkGetSetPixelsFunc was casting the type of lvalue in an assignment
instead of constructing a matching rvalue type.  In this context it
caused the Intel compiler (version 9.1 on Linux) to crash with an
internal compiler error.  Furthermore, the assignment was casting a
pointer-to-function to a different type and then calling it through that
type, which is technically undefined behavior.

This change uses a safer approach to perform the same assignment.