fix build error on macOS 10.14 w/ Xcode 10

<filesystem> is not available in macOS 10.14's SDK.  This change causes the "tests" app to use chdir(), if <filesystem> cannot be found.

Design questions

This project now builds under GNU/Linux in my testing environment. Up…

…dated GNU-Linux_build_instructions.txt to reflect current build requirements. The Windows Clang/C2 builds now also build successfully once again. Guards added to require libpng 1.6.X or later and to provide non-R4 compliant functionality where the <filesystem> API is not available. (I never imagined I'd be adding guards because Visual C++ has functionality that other major vendors do not yet have; it's a very welcome development in terms of everyone's commitment to standards compliance). Various other changes to fix bugs exposed by building with Clang 4.0. This commit will be tagged R4. An R4 branch will be created to fix any bugs or issues with this. The main branch will reflect the development of R5 (the successor to R4).

Fixes to the surface class's rendering and composing operations membe…

…r functions to improve usability (removed origin argument, added default identity matrix value for matrix_2d arguments and made it so that the Mask Brush behaves similar to the source brush).

Revert "Renamed surface::push_group_with_content to surface::push_gro…

…up. Renamed surface::copy_clip_rectangle_list to surface::get_clip_rectangles. Added path_data validation code to path class constructor."

This reverts commit 42a1981.

Added build instructions document for GNU/Linux systems.