Add high precision timer.
System::step now takes delta time as a float instead of an unsigned int.
SystemManager.h: Use high precision timer for calculating frame delta times.
Replace NULL with nullptr.
GraphicsSystem.cpp: Remove code which did nothing.
GraphicsSystem::loadTexture: Take std::string parameter as const ref to prevent needless copying.
GraphicsSystem.h: Remove unneeded #include.
GraphicsSystem::loadTexture: Improve error messages and simplify code.
EventSystem::step: Comment out name of unused parameter.
GraphicsSystem::step: Comment out name of unused parameter.
CMakeLists.txt: Compile everything as 1 project instead of compiling 'engine' as separate library.
Remove Engine/CMakeLists.txt; no longer needed.
Fix include being broken after cmake changes.
GraphicsSystem::loadTexture: Add newline in log message.
CMakeLists.txt: Include headers/source files in Engine/ External/ recursively.
GraphicsSystem: Add post-render callback.
Add GeometryDrawer class.
GraphicsSystem: Add getter for SDL_Renderer.
main(): Add initialization of GeometryDrawer.
GraphicsSystem: Make background color dark blue instead of white.