Adds PNG library to linker list

- Starting to work on issue #2 for saving rendered images as PNG files.
- Linker list includes PNG library.
- C file includes PNG include directive.
Changes functions to remove unused variables

- Unused variables removed.
- Tidies variables.
Adds PNG support and closes #2

- PNG writing supported via “s” hotkey. This closes #2.
Merge branch 'feature/WritePNGs'