Easier platform porting (#108)

* Create PORTING.md

* Creates io_platform abstraction

* removes miniposix

* remove unistd refs

* removes watcom functions

* fixes shadow rendering memory leak

CMakeLists.txt

@@ -20,7 +20,6 @@ option(BUILD_TESTS "Build unit tests." OFF)
 find_package(SDL2 REQUIRED)
 
 add_subdirectory(lib/libsmacker)
-add_subdirectory(lib/miniposix)
 add_subdirectory(lib/glad)
 add_subdirectory(lib/cglm EXCLUDE_FROM_ALL)
 

docs/PORTING.md

@@ -0,0 +1,68 @@
+# Porting Dethrace to other systems
+
+## Operating Systems
+
+See: xxx
+
+Assuming an operating system called _foo_, follow the steps to add support for it.
+
+1. Add a new file `os/foo.h` and implement the required functions defined in `os.h`:
+- `OS_GetTime`
+- `OS_Sleep`
+- `OS_Basename`
+- `OS_GetFirstFileInDirectory`
+- `OS_GetNextFileInDirectory`
+- `OS_IsDebuggerPresent`
+- `OS_InstallSignalHandler`
+
+2. Update `src/harness/CMakeLists.h` and add a new conditional section for "os/foo.h", based on existing conditions for Windows, MacOS etc. 
+
+For example:
+
+```
+...
+elseif( _FOO_ )
+target_sources(harness PRIVATE
+    os/foo.c
+)
+...
+```
+
+## IO Platform (windowing / input / rendering)
+
+An `IOPlatform` in _dethrace_ implements windowing and input handling, and points to a _renderer_.
+
+The default IO platform is `SDL_OpenGL`, which uses SDL for windowing and input, and OpenGL for rendering. See `io_platforms/sdl_gl.c`.
+
+To add a new `IOPlatform`:
+
+1. Create `io_platforms/my_platform.c` file and implement the required functions defined in `io_platform.h`:
+- `Window_Create`
+- `Window_PollEvents`
+- `Window_Swap`
+- `Input_GetKeyMap`
+- `Input_IsKeyDown`
+
+`Window_Create` returns a `tRenderer*`, which must implement the interface defined in `renderers/renderer.h`. See `renderers/gl` for an example.
+
+2. Add a new conditional section in `src/harness/CMakeLists.txt` for your new platform
+
+For example:
+```
+if (IO_PLATFORM STREQUAL "My_Platform")
+    target_sources(harness PRIVATE
+        io_platforms/my_platform.c
+    )
+endif()
+```
+
+3. Run cmake to update your build with the new platform
+```sh
+cd build
+cmake -DIO_PLATFORM=My_Platform ..
+```
+
+4. Build
+```
+cmake --build .
+```