This is a code refactor in preparation of supporting canvas
compositing and fix all cropping issues on full frame implementation.
No functional changes, all canvases are at (0, 0) position matching
tiled implementation.
All operations with constant pixel/elements are evaluated into primitive Value/Vector/Color constant operations prior rendering execution. This is the case of operations that are constant only when all their inputs are so. Such operations should set can_be_constant flag to true. is_constant flag is only used for primitive operations (Color/Vector/Value) which are the only constant operations during rendering execution.