• engine: Add support for constant parameters, which are constants that can be specialized after material compilation.
• materials: improved size reduction of OpenGL/Metal shaders by ~65% when compiling materials with
  size optimizations (matc -S) [⚠️ Recompile Materials]
• engine: fix potential crash on Metal devices with A8X GPU (iPad Air 2) [⚠️ Recompile Materials]
• opengl: support the external image on macOS