Fix build errors with modern compilers

Random improvements

Add test program, fix Linux issues

Update to C++17

Configure render thread for real-time rendering on Apple platforms

Fix compiler warning about deleted constructors

Fix various compilation issues, remove deprecated auto_ptr usage

Node active state and world transform function

Fix totally broken _pixel_frmt_from_sdl_frmt function

Fix build warnings about version-specific libraries

Remove unnecessary debug log

Fix issue where the SDL audio device may not be initialized correctly

Fix compiler warnings, bump dependency versions

Fix compiler warnings

OpenGL extensions, GLES2 support

Beginning work on OpenGLES 2.0 support

Fix rendering glitch

Support for 3D models custom rendering modes

Sequence action

cmake: use module for SDL2_image instead of FindPkgSource